Olefin hydroformylation and kinetic studies using mono- and trinuclear N,O-chelate rhodium(I)-aryl ether precatalysts

Article abstract of DOI:10.1016/j.apcata.2019.117179

Rh(I)-salicylaldimine-triazole mononuclear (6) and trinuclear (7) complexes based on an aryl-ether scaffold were investigated as precatalysts in the rhodium-catalysed hydroformylation of the terminal olefin, 1-octene, and internal olefins, 7-tetradecene and 4-octene. The complexes generally show good catalytic activity in the hydroformylation of 1-octene at temperatures ranging from 75 °C to 95 °C and syngas pressures of 20 to 40 bar. The precatalysts have excellent stability and could be reused several times using organic solvent nanofiltration under optimum conditions of 85 °C and 40 bar. Kinetic studies using catalyst precursor 6 were investigated by evaluating the effect of temperature, syngas total pressure and catalyst loading on the rate of hydroformylation. The activation energy for the hydroformylation of 1-octene was calculated to be 62 kJ mol^?1 and the experimental rate constants were found to be in good agreement with the predicted rate data obtained using a modified fundamental mechanism-based rate model.

Full text of DOI:10.1016/j.apcata.2019.117179

Accepted Manuscript
Title: Olefin hydroformylation and kinetic studies using mono-
and trinuclear N,O-chelate rhodium(I)-aryl ether precatalysts
Authors: Shepherd Siangwata, Nicholas C.C. Breckwoldt,
Neill J. Goosen, Gregory S. Smith
PII:
DOI:
Article Number:
S0926-860X(19)30334-5
https://doi.org/10.1016/j.apcata.2019.117179
117179
Reference:
APCATA 117179
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
30 April 2019
11 July 2019
26 July 2019
Please cite this article as: Siangwata S, Breckwoldt NCC, Goosen NJ, Smith
GS, Olefin hydroformylation and kinetic studies using mono- and trinuclear N,O-
chelate rhodium(I)-aryl ether precatalysts, Applied Catalysis A, General (2019),
https://doi.org/10.1016/j.apcata.2019.117179
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Olefin hydroformylation and kinetic studies using mono- and
trinuclear N,O-chelate rhodium(I)-aryl ether precatalysts
a
b
b
a
Shepherd Siangwata, Nicholas C. C. Breckwoldt, Neill J. Goosen, Gregory S. Smith *
a
Department of Chemistry, University of Cape Town, Rondebosch, 7701, South Africa.
b
Department of Process Engineering, Stellenbosch University, Private Bag X1, Matieland, 7602, South
Africa
*
Corresponding author. Tel.: +27 (0)21 650 5279
E-mail address: gregory.smith@uct.ac.za
Graphical abstract
Highlights




Rhodium-salicylaldimine precatalysts evaluated for olefin hydroformylation.
Catalysts can be reused using organic solvent nanofiltration.
-
1
Both catalysts have an activation of 62 kJ mol .
Observed rate constants compare favourably with a modified rate model.
1

ABSTRACT
Rh(I)-salicylaldimine-triazole mononuclear (6) and trinuclear (7) complexes based on
an aryl-ether scaffold were investigated as precatalysts in the rhodium-catalysed
hydroformylation of the terminal olefin, 1-octene, and internal olefins, 7-tetradecene
and 4-octene. The complexes generally show good catalytic activity in the
hydroformylation of 1-octene at temperatures ranging from 75 ºC to 95 ºC and syngas
pressures of 20 to 40 bar. The precatalysts have excellent stability and could be
reused several times using organic solvent nanofiltration under optimum conditions of
8
5 ºC and 40 bar. Kinetic studies using catalyst precursor 6 were investigated by
evaluating the effect of temperature, syngas total pressure and catalyst loading on the
rate of hydroformylation. The activation energy for the hydroformylation of 1-octene
-
1
was calculated to be 62 kJ mol and the experimental rate constants were found to
be in good agreement with the predicted rate data obtained using a modified
fundamental mechanism-based rate model.
Keywords: hydroformylation; olefins; rhodium; nanofiltration; kinetics
1
. Introduction
The hydroformylation reaction is a metal-catalysed reaction, involving the addition of
carbon monoxide and hydrogen to alkenes to produce aldehydes. This atom efficient
reaction, also known as the “Oxo process” is the largest homogeneous transition metal
complex-catalysed reaction in industry, producing over 14 million tons of aldehydes
annually [1–4]. Subsequent transformation of these aldehydes (linear or branched)
leads to “oxo intermediates” which are further processed into downstream products
mainly used in the pharmaceutical, detergent and agrochemical industries. The widely
used metal of choice is rhodium owing to its good catalytic activity and selectivity for
aldehydes under mild reaction conditions (typically, 80 – 100 ºC and 10 – 25 bar) [5,6].
However, the use of rhodium-based catalyst precursors in hydroformylation, like most
homogeneous catalytic systems, is hampered by separation of the catalyst from the
products. The presence of residual metal in the product stream often comes with huge
financial implications that impacts on several fields. Separation of the metal from the
products through the conventional distillation methods is only suitable for low boiling
point short chain alkenes, since the high temperatures that are required for long chain
alkenes often leads to catalyst decomposition and subsequently low catalyst activity
2

[
7]. Several strategies have been used to facilitate the recovery of homogeneous
catalysts in hydroformylation, while maintaining the high activity and selectivity of the
rhodium-based catalyst precursors. These strategies mainly involve the immobilisation
of a catalyst on a solid support (polymers), as well as heterogenization of a catalyst in
a different phase to that of the reactants and products (biphasic catalysis) [8][9–18]. A
less explored, though highly attractive recovery technique is the use of membrane
technology which allows permeation through the membrane by selected molecules
upon application of a driving force. The permeation of membranes is easily tuneable
by manipulating the various physicochemical properties of both the membrane (pore
size, material, thickness and diameter) and the constituents of the feed (molecular
weight, polarity, geometry, viscosity and surface tension) [19–24]. A noteworthy
strategy in membrane technology is the organic solvent nanofiltration technique
(
OSN), which entails the use of nanofiltration membranes to separate molecules
present in organic solvents without the need for processes that are catalyst-destructive
or that require high-energy [25–32]. This is considered a greener approach towards
homogeneous catalyst recovery because of its relatively low energy requirements and
facile catalyst separation without the need for biphasic media. These membranes are
also deemed user-friendly and the whole process can be readily scaled up. Moreover,
the OSN recovery technique is highly successful when the catalyst is of high molecular
weight relative to the other constituents of the feed. This allows for the determination
of the molecular weight cut-off (MWCO), which is the molecular weight at which the
membrane will achieve 90% rejection/retention [33]. Since homogeneous catalysts
(
such as those used for hydroformylation) are usually of the same molecular weight
as the reagents and products, molecular weight enlargement (MWE) of the catalyst
can facilitate efficient membrane filtration. Various soluble supports have been used
for molecular weight enlargement of homogeneous catalysts, and these include
polyhedral oligomeric silsesquioxanes (POSS), polymers and dendrimers [34–36].
When metals are incorporated into dendritic arms of the support, the resulting
metallodendrimers often possess enhanced catalytic activity due to the multiple
catalytic sites that propagate from the core [37–39]. These mimic naturally occurring
metalloenzymes, which often possess improved catalytic efficiency over their
mononuclear analogues owing to multiple active sites. From a Green Chemistry
perspective, there are huge gains through a catalytic system that can offer excellent
efficiency with facile catalyst recovery for multiple reuse.
3

Herein, we report on the synthesis and application of mono- and trinuclear complexes
of Rh(I) as catalyst precursors in the hydroformylation of a linear α-olefin and internal
olefins. The recovery of these complexes using the OSN technique, as well as their
kinetic studies in the hydroformylation of 1-octene, is evaluated.
2
. Experimental
2
.1 General
All chemicals and solvents were reagent grade and used as received from Sigma-
Aldrich, unless otherwise stated. N-3-Bromopropylsalicylaldimine [40], phenyl
propargyl ether (2) [41] and 1,1,1-tris(4-propargyl ether phenyl)ethane (3) [42] were
prepared according to modified previously reported literature procedures. RhCl
3
.3H O
2
was purchased from Heraeus South Africa (Pty) Ltd.7-Tetradecene was prepared via
the metathesis of 1-octene using the Hoveyda–Grubbs second generation pre-
catalyst, and then purified by removing the metathesis catalyst via organic solvent
nanofiltration. The unreacted 1-octene was then boiled off to yield 7-tetradecene in
high purity (>98%) [43]. Carbon monoxide and hydrogen gas were supplied by Afrox
Ltd. Nuclear Magnetic Resonance (NMR) spectra were recorded on either a Bruker
1
13
1
Biospin GmbH ( H: 400.22 MHz; C: 100.65 MHz) or a Varian XR300 MHz ( H: 300.08
13
MHz; C: 75.46 MHz) spectrometer. NMR values were reported relative to the internal
standard tetramethylsilane (δ 0.00). FT-IR spectra were recorded using Attenuated
Total Reflectance Infrared spectroscopy (ATR-IR). Melting points were determined
using a BÜCHI melting point apparatus B-540. Mass spectrometry was carried out
using a Waters API Quattro Micro Triple Quadrupole electrospray ionisation mass
spectrometer in the positive-ion mode. The mobile phase for LCMS analyses was
prepared using solvents and reagents of HPLC-grade obtained from Sigma-Aldrich
(
(
ammonium acetate as an additive), Merck (glacial acetic acid) and Microsep
acetonitrile and methanol). Low Resolution-ESI-MS was acquired on an Agilent 1260
®
®
Infinity HPLC system (Agilent 1260 Infinity Binary Pump, Agilent 1260 Infinity Diode
®
®
Array Detector (DAD), Agilent 1290 Infinity Column Compartment, and Agilent 1260
Infinity Standard Auto sampler) coupled to Agilent 6120 Quardrupole MS system and
®
®
Peak Scientific Genius 1050 Nitrogen Generator. Phenomenex Kinetex 2.6 µm EVO
C18 100 Å (30 x 2.1 mm) reverse phase analytical column was used. The
4

chromatographic method included a column temperature of 40 ºC, an injection volume
of 2 µL, flow rate of 0.7 mL/ min and maximum column back pressure set at 600 bars.
The mass spectrum was acquired using electrospray ionisation (ESI) in the positive
ionization mode. Analyses and quantification of the catalytic products was carried out
using a Perkin Elmer Clarus 580 GC instrument equipped with a flame ionisation
detector and 30m capillary column. All spectroscopic and analytical data is supplied in
the Supplementary Information.
2
.2 Preparation of azidopropyl salicylaldimine (1).
NaN
bromopropylsalicylaldimine (1.03 g, 4.23 mmol) in DMF (5 mL) [44]. The reaction was
heated under N at 80 °C overnight. The reaction was then allowed to cool to room
3
(0.303 g, 4.66 mmol) was added to a stirring solution of N-3-
2
temperature and then filtered by gravity. The solvent was removed azeotropically
using toluene. The resultant brown crude product was then re-dissolved in ethyl
acetate and washed with water (2 x 50 mL) and then with brine solution (3 x 50 mL).
The organic layer was collected and dried over magnesium sulfate and filtered by
gravity. The solvent was removed from the filtrate to produce 1 as a yellow oil which
was dried in vacuo.
2
.3 Preparation of 4-(phenoxymethyl)-1H-1,2,3-triazol-1-yl-propylsalicylaldimine
ligand (4).
-
2
-2
-2
CuI (1.41 x 10 g, 7.41 x 10 mmol) was stirred in DIPEA (1.91 x 10 g, 0.148 mmol)
at room temperature for 5 minutes in a 10 mL round bottom flask under N [45]. A
2
mixture of the alkyne phenyl propargyl ether 2 (0.196 g, 1.48 mmol) and the
azidopropyl salicylaldimine 1 (0.303 g, 1.48 mmol) was added, and the reaction was
left stirring at room temperature under N
mixture was diluted with dichloromethane (3 mL) to bring the paste into solution and
filtered by gravity. The filtrate was washed with saturated NH Cl (3 x 50 mL) and the
2
, forming a paste. After 24 hours, the reaction
4
organic layer was then dried over magnesium sulfate and filtered by gravity. The filtrate
was reduced to a minimum (ca. 2 mL) and added dropwise into rapidly stirring diethyl
ether (400 mL), yielding the product as a dull yellow precipitate. Vigorous stirring was
5

continued for 1 hour to triturate the product, which was collected by vacuum filtration
and washed with diethyl ether.
2
.4
Preparation
of
1,1,1-tris(4-phenoxymethyl)ethane-1H-1,2,3-triazol-1-yl-
propylsalicylaldimine ligand (5).
-
2
-2
-2
CuI (1.14 x 10 g, 5.98 x 10 mmol) was stirred in DIPEA (1.54 x 10 g, 0.119 mmol)
at room temperature for 5 minutes in a 10 mL round bottom flask under N [45]. The
2
alkyne 1,1,1-tris(4-propargyl ether phenyl)ethane 3 (0.168 g, 0.398 mmol) was
dissolved in dichloromethane (2 mL) and introduced into the flask, and the mixture
was allowed to stir for a further 10 minutes, forming a paste. Azidopropyl
salicylaldimine 1 (0.247 g, 1.21 mmol) was added and the reaction mixture was stirred
at room temperature for 72 hours under N
2
. Dichloromethane (5 mL) was added, and
the mixture was stirred at 45 °C for 1 hour under N
2
. The reaction was allowed to cool
to room temperature and the mixture was filtered by gravity. The filtrate was collected
and filtered through a pad of Celite. The solvent was reduced to a minimum (ca. 2 mL)
and added dropwise into rapidly stirring diethyl ether (400 mL) yielding the product as
a dull yellow precipitate. Vigorous stirring was continued for a further 30 minutes and
the product was collected by vacuum filtration and washed with diethyl ether.
2
.5 Preparation of 4-(phenoxymethyl)-1H-1,2,3-triazol-1-yl-propylsalicylaldimine-
based Rh(I) complex (6)
The rhodium(I) complexes were prepared following a method reported previously [46].
-
2
Triethylamine (1.37 x10 g, 0.135 mmol) was added to a stirring solution of the 4-
-
2
(
phenoxymethyl)-1H-1,2,3-triazol-1-yl-propylsalicylaldimine ligand 4 (3.50 x 10 g,
0
.104 mmol) in dichloromethane (5 mL) and the reaction was left stirring for 30
-
2
-2
minutes. A half molar equivalent of [RhCl(COD)] (2.56 x 10 g, 5.20 x 10 mmol) was
2
added and the reaction was left to stir for 24 h at room temperature. The solution was
diluted with dichloromethane (15 mL) and washed with distilled water (2 x 30 mL). The
organic phase was collected and dried over magnesium sulfate, filtered by gravity and
the solvent in the filtrate was reduced to a minimum (ca. 2 mL). Diethyl ether was
6

added (15 mL) to precipitate the product as a yellow solid which was collected by
vacuum filtration and dried in vacuo.
2
.6
Preparation
of
1,1,1-tris(4-phenoxymethyl)ethane-1H-1,2,3-triazol-1-yl-
propylsalicylaldimine-based Rh(I) complex (7).
-
2
Triethylamine (2.88 x 10 g, 0.285 mmol) was added to a stirring solution of the 1,1,1-
tris(4-phenoxymethyl)ethane-1H-1,2,3-triazol-1-yl-propylsalicylaldimine ligand 5 (7.55
-
2
-2
x 10 g, 7.31 x 10 mmol) in dichloromethane (5 mL) and the reaction was left stirring
-
2
for 30 minutes [46]. A one and a half molar equivalent of [RhCl(COD)]
.110 mmol) was added and the reaction was left to stir for 24 h at room temperature.
The solution was diluted with dichloromethane (15 mL) and washed with distilled water
2 x 30 mL). The organic phase was collected and dried over magnesium, filtered by
2
(5.40 x 10 g,
0
(
gravity and the solvent in the filtrate was then reduced to a minimum (ca. 2 mL). Diethyl
ether was added (15 mL) to precipitate the product as a yellow solid which was
collected by vacuum filtration and dried in vacuo.
2
.7 General procedure for the catalytic experiments
Hydroformylation studies were conducted in a 100 mL stainless steel pipe reactor
equipped with a Teflon interior. In a typical experiment, the reactor was charged with
toluene (7.5 mL), 1-octene (1.21 g, 10.7 mmol), dodecane as the internal standard
(
100 µL) and 6 (2.87 × 10–4 mmol, substrate : Rh ratio 2500 : 1). The reactor was
heated to the desired temperature, flushed with syngas (CO:H = 1:1) and then
2
pressurised to the appropriate syngas pressure. Samples were analysed after 4 hours
using gas chromatography (GC). Authentic iso-octenes and aldehydes, alcohols and
n-octane were used to confirm the products. Recyclability experiments were carried
out after 2 hours using the organic solvent nanofiltration technique (OSN) under the
optimised conditions of temperature and pressure. In a typical recovery experiment,
after the first catalytic run, the reaction components were cooled to room temperature,
and diluted with toluene (7.5 mL). The solution was emptied into a stainless-steel
®
dead-end separation cell mounted with a pre-conditioned Duramem 200 membrane,
and the cell was pressurised to 25 bar with nitrogen gas to enable the feed to flow
7

through the membrane. The permeate was collected over time and the cell was de-
pressurised when the feed was reduced to a minimum. The retentate containing the
catalyst was decanted from the cell and re-introduced into the catalytic reactor bearing
fresh substrate and toluene. The catalytic reaction was repeated under the same
conditions of temperature, pressure and time, and subjected to the OSN technique
after each cycle. Kinetic studies on the hydroformylation of 1-octene were conducted
using the mononuclear catalyst precursor 6 through varying the temperature, time,
catalyst loading and total syngas pressure.
3
. Results and discussion
3
.1 Synthesis of salicylaldimine-triazole-based mono- and trimeric ligands (4 and 5).
The precursor, azidopropyl salicylaldimine ligand (1), was prepared by the reaction of
N-3-bromopropylsalicylaldimine [40], with sodium azide in dimethylformamide to
obtain the product as a yellow oil in good yield (89%), (Scheme 1). The precursors to
the salicylaldimine-triazole ligands 4 and 5 were prepared via a Williamson ether
reaction of propargyl bromide with either phenol or THPE in acetone respectively. The
resultant aryl-ether alkyne products were isolated as either a yellow oil (2), or a white
solid (3). Subsequently, the new yellow aryl-ether triazole-based ligands 4 and 5 were
prepared in good yields (85% (4) and 90% (5)) via a diisopropylethylamine-promoted
CuI-catalysed azide-alkyne cycloaddition reaction of azidopropyl salicylaldimine 1 with
1
the respective alkyne (2 or 3) (Scheme 1). The H NMR spectra of 4 and 5 show
diagnostic triazole signals at ca. δ = 8.25 (H12), and the imine proton signals (H ) at
8
ca. δ = 8.50 for both compounds (Fig. S1 and Fig. S2). The diagnostic methyl signal
of the trimeric core of 5 is observed upfield at δ = 2.04. The infrared spectra confirm
-
1
formation of 4 and 5 by the presence of absorption bands at ca. 3085 cm , assigned
to the ν(C–H) stretching frequency of the 1,2,3-triazole and characteristic strong imine
-
1
ν(C=N) absorption bands at ca. 1630 cm for both ligands. Additionally, the spectra
do not show the absorption bands previously reported for the azido (1) [40] and alkyne
(
2 and 3) [40 41] stretching frequencies. This is in line with data reported in the
,
literature for Cu(I)-catalyzed azide-alkyne “click” cycloaddition reactions [47].
8

3
.2 Synthesis of aryl-ether Rh(I)-salicylaldimine-triazole-based mono- and trinuclear
complexes (6 and 7).
The new aryl-ether Rh(I)-salicylaldimine-triazole-based complexes 6 and 7 were
synthesised by deprotonation of the ligands 4 and 5 using triethylamine, and
subsequent complexation with the appropriate molar equivalent of the Rh(I) dimer
[
RhCl(COD)]
2
, (COD = 1,5-cyclooctadiene), [46] Scheme 1. The products were
1
isolated as brown solids in good yields (83% (6), and 99% (7)). The H NMR spectra
(
Fig. S3 and Fig. S4) show upfield shifts of the imine proton signals from ca. δ = 8.50
in 4 and 5 to δ = 8.18 in 6 and 7, characteristic of the synergistic effect involving M to
L pi-donation and L to M sigma donation. The infrared spectra of both complexes also
show shifts of the imine absorption bands to lower wavenumbers upon coordination of
the ligands to the metal, corroborating the coordination of the imine nitrogen. This has
been reported for similar compounds in the literature [48–51].
3
.3 Preliminary screening using precatalyst 6
Preliminary experiments were performed using the model mononuclear complex (6)
as the catalyst precursor, and 1-octene as the substrate. The effects of temperature
(
75 – 95 °C) and pressure (20 – 40 bar) were evaluated in the hydroformylation of 1-
octene over 4 hours. Moderate conversion of 1-octene (79%) is observed at the lowest
temperature and pressure (75 °C and 20 bar syngas pressure) of this study (Fig. S5).
Increasing the temperature (to 85 °C and 95 °C) at a constant pressure of 20 bar
results in near-quantitative conversion of 1-octene (>97%), indicative of the energy
required for activation of the olefin. A further increase in temperature and pressure has
no significant effect on the conversion of 1-octene. A gradual increase in
chemoselectivity towards aldehydes is observed with an increase in temperature at
constant pressure (20 bar, 75 °C – 95 °C), Figure S6. A further increase in pressure
with an increase in temperature results in improved chemoselectivity towards
aldehydes of up to 96%. The observed bias towards aldehydes with an increase in
syngas pressure and temperature can be ascribed to the hydroformylation of the iso-
octenes to aldehydes due to increased CO concentration in solution which limits
isomerisation and accelerates the CO migration step [52]. A higher degree of linear
9

aldehydes (73%) is observed at low conditions of temperature (75 °C) and pressure
20 bar), Figure S7, whereas increasing the temperature and pressure results in a
(
decrease in linear aldehydes, as there exists preferential isomerisation of the terminal
olefin at high temperatures, leading to iso-octenes which gradually undergo
hydroformylation to form branched aldehydes. This observation is further supported
by the n:iso ratios in Table 1, which decrease as the temperature is raised from 75 °C
to 95 °C. It can be postulated that the tetracarbonyl rhodium species [Rh(CO)
]+ may
4
form at higher pressure, and can influence the selectivity of the catalyst. The increase
in pressure at constant temperature results in a steady increase in activity from 154 h-
1
-1
(
75 °C/ 20 bar) to 444 h (75 °C/ 40 bar). Increasing the temperature to 85 °C at a
-
1
syngas pressure of 40 bar gives the best activity of 1-octene (554 h , entry 6).
3
.4
Mercury poisoning experiment
The mercury drop test was carried out to determine if the reaction was entirely
homogeneous or may have taken place in the presence of nanoparticles (entry 7,
Table 1). A negligible decrease in conversion was observed {from 99% (entry 6) to
9
3% (entry 7)}, suggesting that it is largely a molecular species that catalyses the
reaction and that nanoparticles may contribute to a small extent to the observed
transformation of the olefin to the aldehydes, as has been observed in previous studies
with similar Rh(I) catalyst precursors [44,47].
3
.5
Catalytic evaluation using precatalyst 7
From the optimisation experiments using catalyst precursor 6, the best conditions were
established at 85 °C and 40 bar, based on the good chemoselectivity for aldehydes
(
90%), near-quantitative conversion of the substrate (99%) and the high activity (554
-
1
h ). As a consequence of the near complete conversion and to delineate more
meaningful results, it was decided to evaluate the trinuclear precatalyst at a reduced
syngas pressure. The trinuclear complex (7) was successfully tested in the
hydroformylation of 1-octene under milder reaction conditions of 85 °C and 20 bar.
The dendritic complex (7) shows a similar catalyst performance as the mononuclear
analogue (6) under the chosen conditions of study (Table 2). It is worth noting that the
trinuclear complex is less soluble in toluene than the mononuclear complex under the
10

tested conditions, suggesting a less favourable interaction between the substrate and
the catalyst precursor. Previous literature studies report that the poorer solubility of
dendritic catalyst precursors in the conventional solvent of choice for hydroformylation
(
toluene), sometimes lead to less evident dendritic effects [49,50].
3
.6
Substrate variation
The substrate scope was extended to the internal olefins 7-tetradecene and 4-octene,
which are highly important substrates in the surfactant industry. Hydroformylation of
such substrates is generally carried out at high temperatures (>100 °C) owing to the
high thermodynamic stability of internal olefins over their terminal counterparts [55–
5
7]. In this study, the experiments were carried out at 120 °C and 40 bar for 4 hours
using the mononuclear complex 6 (Table 3).
Good conversions of both substrates were observed (>80%), with catalyst precursor
6
showing good chemoselectivity towards aldehydes (90%) for 7-tetradecene. The
relatively shorter carbon chain internal olefin trans-4-octene gave a higher percentage
of iso-octenes to aldehydes (82% : 18%, respectively), which could be explained by
the relative ease of isomerisation of trans-4-octene compared to 7-tetradecene, which
is highly thermodynamically stable. Moreover, minute quantities of 1-octene and
nonanal were observed in the hydroformylation of trans-4-octene, indicative of the
highly preferential isomerisation of trans-4-octene over hydroformylation.
3
.7 Reusability of the catalyst: Organic Solvent Nanofiltration
Catalyst recovery and recyclability was carried out for 2 hours using the organic
solvent nanofiltration technique at 85 °C and 40 bar. The separation was achieved by
using a stainless-steel dead-end separation cell mounted with a pre-conditioned
®
Duramem 200 membrane. The choice of the membrane was mainly influenced by
-
1
the molecular weight cut-off of the membrane (MWCO = 200 g mol ), in-line with the
-
1
molecular weight of the catalysts under study (complex 6 = 546.48 g mol ; complex 7
-
1
=
1447.35 g mol ). The catalyst precursors 6 and 7 maintain good conversion of 1-
octene after OSN recovery with each cycle (Fig. 1), with catalyst precursor 6 showing
11

th
only a slight decline in conversion (to 91%) after the 4 cycle, indicative of the
effectiveness of the OSN membrane in recycling the catalysts. Most remarkably, the
catalyst precursors 6 and 7 show good catalytic activity with each cycle (Fig. 2). Both
catalyst precursors 6 and 7 show appreciably good to moderate chemoselectivity for
aldehydes with each cycle (Fig. 3). The preference for aldehydes is comparable for
both catalyst precursors with each cycle, with the best aldehyde chemoselectivity at
7
8% (Run 2 for both complexes). Beyond the 2nd cycle, the chemoselectivity for
aldehydes is observed to decline slowly, ascribed to the formation of iso-octenes being
favoured at this stage. This suggests that the active species remains the same with
each recycle and some degradation may occur, leading to the slight decline in
chemoselectivity.
3
.8
Product-distribution-time studies using 6: Influence of temperature
The influence of temperature on the product distribution over time was investigated
during the hydroformylation of 1-octene at constant pressure of 40 bar using catalyst
precursor 6 (Fig. S8). The reactions were carried out over a maximum period of 4
hours, and the reported values for 1-octene represent the number of moles of
substrate that are remaining at each respective time interval. At the lowest
temperature of this study (75 °C), the number of moles of the substrate decrease
steadily over time, Fig. S8a. Increasing the temperature to 85 °C shows a substantial
decline of 1-octene in the system over time, with a further increase in temperature to
9
5 °C resulting in near-perfect depletion of the substrate. Moreover, at a lower
temperature of 75 °C, a steady build-up of iso-octenes is observed between 1 hour
and 3 hours of the reaction, Fig. S8c. This would be expected as the rate of
isomerisation is generally slower at lower temperatures [51,54]. However, as these
iso-octenes build-up in the system at 75°C, an optimum level is reached at 3 hours,
beyond which a driving force towards conversion of the isomers to branched
aldehydes comes into effect, resulting in the observed rapid decline in iso-octenes,
from 31.21 mol% to 8.25 mol% at 3 and 4 hours respectively (Fig. S8c). This
observation is further supported by the sharp increase in branched aldehydes between
3
and 4 hours of the reaction at 75 °C, from 11.57 mol% to 31.64 mol% respectively,
Fig. S8d. At higher temperatures of 85 °C and 95 °C, the rate of isomerisation is
12

greater immediately from the 1-hour interval (Fig. S8c), a phenomenon that is
expected at high temperatures. These iso-octenes decline rapidly as they are
immediately converted to branched aldehydes, further substantiating the observed
trend depicted in Fig. S8d at the same temperatures of 85 °C and 95 °C. Moreover,
the formation of the linear aldehyde (nonanal) with increase in temperature occurs
slowly with time (Fig. S8b), as the dominance of the branched aldehydes becomes
pronounced due to the high temperatures favouring the double-bond isomerisation of
the substrate prior to hydroformylation.
3
.9
Product-distribution-time studies using 6: Influence of pressure
The influence of pressure on the product distribution over time was investigated during
the hydroformylation of 1-octene at constant temperature of 85 °C using catalyst
precursor 6 (Fig. S9). It should be mentioned that there was no notable influence on
performance of the catalyst when the partial pressure of the CO and H
at 85 °C, hence the data reported herein represents that obtained at equimolar ratio
of CO:H . The lower syngas pressure of 20 bar favours greater accumulation of iso-
2
were varied
2
octenes with time (over two hours) before these can build-up sufficiently enough to
create a driving force for their hydroformylation into aldehydes (after 2 hours, Fig. S9c).
This driving force is reached earlier at higher pressures (at 1 hour, for 30 and 40 bar,
Fig. S9c), allowing for a more improved transformation of these iso-octenes to
branched aldehydes between 1 and 4 hours of reaction time. This corresponds well
with the observed sudden increase of branched aldehydes from 1 hour of reaction time
(
Fig. S9d) at the same syngas pressure of 30 and 40 bar.
3
.10 Product-distribution-time studies using 6: Influence of catalyst loading
The influence of catalyst loading on the product distribution over time was investigated
during the hydroformylation of 1-octene at the optimum temperature (85 °C) and
pressure (40 bar) using catalyst precursor 6 (Fig. S10). The quantity of 1-octene in the
system is observed at comparable levels to the normal and double catalyst loading
between 2 hour and 4 hour time intervals (Fig. S10a), which is indicative of a system
that has reached the optimum point of influence with respect to the catalyst loading.
Interesting to note is the increase in iso-octenes from 1 hour (18.27 mol%) to 2 hours
(
37.87 mol%) at half catalyst loading (Fig. S10c), indicative of the influence of low
13

catalyst concentration on lowering the rate of isomerisation. This leads to a slower
build-up of iso-octenes, before finally being overcome at a longer reaction time (2–4
hours) via conversion thereof to the branched aldehydes, which is evidently later than
that at higher catalyst concentrations, an observation further supported by the profile
depicted in Fig. S10d.
A closer look at the chemo- and regioselectivity product distribution profiles using the
normal catalyst loading under optimum conditions (85 °C and 40 bar) is shown in Fig.
4
. The iso-octenes accumulate during the first hour of reaction, beyond which
hydroformylation occurs rapidly, leading to formation of more branched aldehydes.
3
.11 Product-distribution-time studies using 7
The product-distribution-time profile using catalyst precursor 7 for the hydroformylation
of 1-octene were evaluated and compared against that of the mononuclear complex 6
at a temperature of 85 °C and a syngas pressure of 20 bar. The kinetics of the
trinuclear complex are similar to that of the mononuclear complex (Fig. 5). This
behaviour suggests that the metal centres of the trinuclear complex are behaving as
a mononuclear entity in the system, which correlates with the catalytic data obtained
(
vide supra). It is generally accepted that similar reaction kinetics translate to similar
rate constants, necessitating the determination of the rate constants of the pre-
catalysts based on the data obtained for the mononuclear complex 6.
3
.12 Determination of the reaction rate constants and calculation of activation energy
From the product distribution profiles of the catalyst precursor 6 (based on total
aldehydes formed), the observed reaction rate constants (kobs) were then estimated
from equation 1 using the least square regression analysis, with first-order
dependence in the olefin, shown in Table 4 below.
푑(퐶표푐푡푒푛푒푠
푑ꢀ
)
=
−푘표푏푠퐶표푐푡푒푛푒푠
(1)
14

The rate constants agree with the general expectation of the rate of a catalytic reaction
increasing with increase in temperature at constant pressure (40 bar, entries 1 to 3).
Moreover, at constant temperature (85 °C), varying the pressure results in an increase
in the rate constants, indicative of the effect of increased concentration of syngas in
-
1
-1
the system, from 0.13 h (20 bar, entry 5) to 0.49 h (40 bar, entry 2). The effect of
the catalyst loading is also evident on the rate constants at a constant temperature
-
1
and pressure of 85 °C and 40 bar respectively, increasing from 0.39 h (entry 6, half
-
1
-1
catalyst loading) to 0.49 h (entry 2, normal catalyst loading) and 0.71 h (entry 7,
double catalyst loading). As expected, the influence of temperature on the rate
constant is consistent with the Arrhenius Equation (Equation 2).
퐸
푅푇
ꢂꢃ
ꢄ
푘표푏푠 = 푘0ꢁ
(2)
-
1
From this equation, the activation energy was calculated to be 62 kJ.mol . This value
is comparable to the ranges obtained using similar rhodium(I) catalysts for the
hydroformylation of medium-chain alpha-olefins in a homogeneous solution [59–61].
To further describe the observed rate constants (kobs) in terms of catalyst and syngas
pressure effects of Table 4, a fundamental mechanism-based rate law model was
applied, as proposed by Dutta and co-workers [61], and shown in Equation 3.
푑퐶표푐푡푒푛푒푠
푑ꢀ
푘퐶 (푃 )(푃 )퐶
푅ℎ ꢅ푂 퐻ꢆ 8
=
−
(3)
1 + 퐾(푃ꢅ푂)
However, the rate model did not satisfactorily describe the observed rate constants.
This might be due to simplifications of the mechanistic steps used to derive the rate
model equation. To obtain a better fit of the observed rate data necessitated modifying
the rate model with respect to the catalyst loading, as given in Equation 4.
훼
푑퐶표푐푡푒푛푒푠
푑ꢀ
푘퐶 (푃 )(푃 )퐶
푅ℎ ꢅ푂
퐻ꢆ
8
=
−
(4)
1 + 퐾(푃ꢅ푂)
A summary of the kinetic parameters obtained from Equation 4 are shown in Table 5.
15

A parity plot of the observed and predicted rate constants was then generated and is
shown in Fig. 6.
The parity plot shows that the observed rate constants satisfactorily compare to the
calculated mechanism-based model rate constants, in support of the experimental
kinetics obtained for the mononuclear complex 6 in the hydroformylation of 1-octene.
To the best of our knowledge, there are no reports of kinetic studies on 1-octene using
salicylaldimine-based Rh(I) complexes. However, several kinetics on hydroformylation
of olefins have been reported in the literature, for both homogeneous and biphasic
systems [62–69].
4
. Conclusions
New salicylaldimine aryl ether 1,4-substituted 1,2,3-triazole-based complexes have
been synthesised. The mononuclear complex 6 was evaluated for optimisation studies
in the hydroformylation of 1-octene. Optimum conditions were established at 85 °C,
4
0 bar for 4 hours. Conversion of the iso-octenes to aldehydes accounts for the
regioselectivity towards branched aldehydes at high temperatures. Mercury poisoning
experiments using catalyst precursor 6 revealed a nanoparticle mediated
transformation of 1-octene. The trinuclear complex posted comparable catalytic
conversion and activity to its mononuclear analogue, possible due to the poor solubility
of the trinuclear complex in toluene. The mononuclear complex was also evaluated in
the hydroformylation of internal olefins 7-tetradecene and 4-octene, with 7-
tetradecene giving good chemoselectivity for aldehydes. The complexes 6 and 7 were
also evaluated for their recoverability in the hydroformylation reaction of 1-octene
using organic solvent nanofiltration (OSN) membrane technology. The complexes
could be recycled for at least 5 times with appreciably consistent catalyst performance
throughout the cycles, which is significant towards addressing one of the major
challenges of designing recoverable and reusable homogeneous hydroformylation
catalysts. Kinetic studies were also carried out to determine the rate constants of the
hydroformylation of 1-octene using catalyst precursors 6 and 7, with catalyst precursor
7
posting similar reaction kinetics to catalyst precursor 6. The reaction rate constants
were then calculated based on the mononuclear complex 6 and the activation energy
16

-
1
was determined to be 62 kJ mol . The observed reaction rate constants correlated
well with a modified fundamental mechanistic-based rate model, validating the
experimental kinetics obtained for the mononuclear complex 6 in the hydroformylation
of 1-octene.
Acknowledgements
We would like to thank the University of Cape Town (UCT), Stellenbosch University,
The Department of Science and Technology of South Africa and NRF-DST Centre of
Excellence in Catalysis – c*change for financial support.
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20

Scheme 1. Synthetic route to aryl-ether Rh(I) mono- and trinuclear complexes 6 and
7
.
21

Conversion
9797 9899
99 99
100
98 98
97
91
80
6
0
complex 6
complex 7
4
0
20
0
Run 1
Run 2
Run 3
Run 4
Run 5
Fig. 1. Percentage conversion of 1-octene during OSN recyclability studies in the
hydroformylation of 1-octene.
Activity
1
000
00
00
00
929
960 957
940
882
920
902
881
8
837
782
6
complex 6
complex 7
4
200
0
Run 1
Run 2
Run 3
Run 4
Run 5
Fig. 2. Activity of the complexes during OSN recyclability studies in the
hydroformylation of 1-octene.
22

Chemoselectivity for aldehydes
100
73
77
7877
80
76
74
74
72
6968
60
complex 6
complex 7
4
0
2
0
0
Run 1
Run 2
Run 3
Run 4
Run 5
Fig. 3. Chemoselectivity for aldehydes during OSN recyclability studies.
Fig. 4. Substrate and product-distribution-time profile at normal catalyst loading (2.87
−3
×
10 mmol of Rh catalyst) and optimum conditions (85 °C /40 bar) with mononuclear
complex 6.
Fig. 5. Substrate and product-distribution-time profiles with the mono- (6) and
trinuclear (7) precatalysts over time.
23

Fig. 6. Parity plot of predicted (kcalc) and experimental (kobs) reaction rate constants.
Table 1 Hydroformylation of 1-octene using catalyst precursor 6 for 4 h.a
n/isoc
TOF (h-1)d
Entry
Temp. (°C)
Pressure
bar)
Conv. (%) Aldehydes Isoalkenes
(
(%)b
31
54
77
44
74
90
56
49
79
89
(%)
69
46
23
56
26
10
44
51
21
11
1
2
3
4
5
6
7e
8
9
75
75
75
85
85
85
85
95
95
95
20
30
40
20
30
40
40
20
30
40
79
96
92
97
98
99
93
99
99
99
73:27
67:33
55:45
49:51
46:54
44:56
63:37
39:61
39:61
40:60
154
323
444
264
454
554
657
301
489
549
10
−
3
aReactions carried out with syngas (1:1) in toluene (7.5 ml) with 7.175 mmol of 1‐octene and 2.87 × 10 mmol of
Rh catalyst. The reactor was purged with syngas. GC conversions obtained using dodecane as an internal standard
b
in relation to authentic standard iso‐octenes and aldehydes. Total aldehydes formed (from octene and iso-alkenes
c
converted) which includes the primary aldehyde product, nonanal, and isoaldehydes. The molar ratio of primary
linear aldehyde product (n) and isoaldehydes (iso) formed. dTOF = (mmol of aldehydes per mmol of Rh)/time.
e
Entry 7 was conducted in the presence of mercury. Average error estimate = ± 0.68.
24

Table 2 Hydroformylation of 1-octene with catalyst precursors 6 and 7 for 4 h.a
n/isoc
TOF (h-1)d
Complex Temp. (°C)
Pressure
bar)
Conv. (%) Aldehydes Isoalkenes
(
(%)b
(%)
56
6
7
85
85
20
20
97
98
44
49:51
49:51
264
265
43
57
−
3
a
Reactions carried out with syngas (1:1) in toluene (7.5 ml) with 7.175 mmol of 1‐octene and 2.87 × 10 mmol of
Rh catalyst for 6, and 9.57 × 10 mmol of Rh catalyst for 7. GC conversions obtained using dodecane as an
internal standard in relation to authentic standard iso‐octenes and aldehydes. Total aldehydes formed (from
octene and iso-olefins converted) which includes the primary aldehyde product, nonanal, and isoaldehydes. The
−4
b
c
d
molar ratio of primary linear aldehyde product (n) and isoaldehydes (iso) formed. TOF = (mmol of aldehydes per
mmol of Rh)/time. Average error estimate = ± 0.76.
Table 3 Hydroformylation of internal olefins with catalyst precursor 6 for 4 h.a
-
1 c
Substrate
Temp.
°C)
Pressure
(bar)
40
Conv.
(%)
85
Aldehydes
(%)b
Isoalkenes TOF (h )
(%)
(
7-tetradecene
120
120
90
10
82
841
102
Trans-4-octene
40
92
18
−
3
aReactions carried out with syngas (1:1) in toluene (7.5 ml) with 7.175 mmol of 1‐octene and 2.87 × 10 mmol of
Rh catalyst for 6. GC conversions obtained using dodecane as an internal standard in relation to authentic standard
b
iso‐octenes and aldehydes. Total aldehydes formed (from substrate and iso-olefins converted) which includes the
c
primary branched aldehyde product and iso-aldehydes. TOF = (mmol of aldehydes per mmol of Rh)/time. Average
error estimate = ± 0.51.
Table 4 Reaction rate constants of catalyst precursor 6.
-
1
Entry
Temp. (°C)
P
Total (bar)
Catalyst load
Normal
Normal
Normal
Normal
Normal
Half
kobs (h )
1
2
3
4
5
6
7
75
85
95
85
85
85
85
40
40
40
30
20
40
40
0.19
0.49
0.60
0.33
0.13
0.39
0.71
Double
−3
Normal catalyst loading = 2.87 × 10 mmol of Rh catalyst, half catalyst loading = 1.44 × 10−3 mmol of Rh catalyst,
and double catalyst loading = 5.74 × 10−3 mmol of Rh catalyst.
Table 5 Summary of the rate model parameters.
Parameter
Value
α
-α
2
-1
5.64 x 108
62
k (L mol bar h )
0
-
1
E (kJ.mol )
A
-
1
K (bar )
0.12
25

α
0.7
26