N′N′N pincer and N′N bidentate(pyrazolylpyridyl) Rh(I) complexes as catalyst precursors for hydroformylation of olefins

Article abstract of DOI:10.1007/s11243-019-00350-2

Abstract: The industrial process of hydroformylation or the oxo process has been used for many years in the production of aldehydes from alkenes. Different metals have been used as efficient catalysts for hydroformylation, in which linear and branched aldehydes are the products obtained; therefore, the development of new catalysts for hydroformylation with high selectivity to aldehydes is important. Rhodium complexes 6–9 were synthesized using [RhCl(CO)₂]₂, or [RhCl(COD)]₂, with either pyrazolylpyridyl N′N′N pincer ligands or a pyrazolylpyridyl N′N ligand. These complexes were then evaluated as catalyst precursors in the hydroformylation reaction using a variety of alkenes. The catalysts all showed activity in hydroformylation but the most active catalyst was methyl-substituted pyrazolyl–rhodium complex 7 following optimization of temperature, syngas pressure and amount of catalyst. Other olefinic substrates were used for hydroformylation in the presence of 7 under the optimum hydroformylation conditions. Undecene and dodecene as substrates only showed minimal formation of aldehydes with predominantly isomerization of the alkene being observed.

Full text of DOI:10.1007/s11243-019-00350-2

Transition Metal Chemistry
https://doi.org/10.1007/s11243-019-00350-2
N′N′N pincer and N′N bidentate(pyrazolylpyridyl) Rh(I) complexes
as catalyst precursors for hydroformylation of olefns
Noluthando V. Gamede1 · Tsitsi A. Kapfunde1 · Edward Ocansey1 · Denis M. Ngumbu1 · James Darkwa1 ·
Banothile C. E. Makhubela1
Received: 6 June 2019 / Accepted: 9 August 2019
© Springer Nature Switzerland AG 2019
Abstract
The industrial process of hydroformylation or the oxo process has been used for many years in the production of aldehydes
from alkenes. Diferent metals have been used as efcient catalysts for hydroformylation, in which linear and branched
aldehydes are the products obtained; therefore, the development of new catalysts for hydroformylation with high selectivity
to aldehydes is important. Rhodium complexes 6–9 were synthesized using [RhCl(CO)2]2, or [RhCl(COD)]2, with either
pyrazolylpyridyl N′N′N pincer ligands or a pyrazolylpyridyl N′N ligand. These complexes were then evaluated as catalyst
precursors in the hydroformylation reaction using a variety of alkenes. The catalysts all showed activity in hydroformylation
but the most active catalyst was methyl-substituted pyrazolyl–rhodium complex 7 following optimization of temperature,
syngas pressure and amount of catalyst. Other olefnic substrates were used for hydroformylation in the presence of 7 under
the optimum hydroformylation conditions. Undecene and dodecene as substrates only showed minimal formation of alde-
hydes with predominantly isomerization of the alkene being observed.
Introduction
fragrance industry. Such aldehydes include, 1-methyldeca-
nal, the aldehyde obtained by hydroformylation of 1-decene
and 2-methylundecanal obtained from condensation of unde-
canal with formaldehyde followed by hydrogenation [5]. In
addition, hydroformylation has been used as an alternate
route in the production of an insecticide, Fenvalerat, through
rhodium-catalyzed hydroformylation of a precursor alkene
then subsequent oxidation to yield an intermediate which is
then used coupled via an ester bond [6].
Homogenously catalyzed hydroformylation serves one of the
most important routes for catalytic processes on an industrial
scale. The products of the hydroformylation reaction are eas-
ily converted into other equally important compounds such
as esters, alcohols and carboxylic acids. These compounds
have been used in making industrially important materi-
als such as plasticizers, solvents and fne chemicals [1–4].
For example, in the production of Vitamin A, 1,2-diace-
toxy-3-butene is hydroformylated via a rhodium-catalyzed
method and produces a branched aldehyde. This intermedi-
ate aldehyde is used in subsequent reactions which include
elimination of acetic acid and a Wittig reaction to yield the
product. Hydroformylation is also applied in the synthesis
of aldehydes which are used to perfume formulations in the
However, in spite of progress made in the hydroformyla-
tion reaction, there have been barriers that need to be over-
come. These include efcient enantioselective hydrofor-
mylation which presently does not contribute signifcantly
towards the formation of chiral aldehydes for industrial pro-
cesses. Currently, practical applications towards such enanti-
oselective hydroformylation is limited by a poor understand-
ing of reaction mechanism as well as the control of factors
leading to control of reaction rate and selectivity [7–9].
In carrying out the hydroformylation reaction, the two
most commonly used systems are the rhodium- and cobalt-
based systems. Rhodium catalysts are favoured over cobalt-
based catalysts, because rhodium allows for the formation of
typically 100–1000 times more active catalysts than cobalt.
In addition, cobalt catalysts generally require high pressures
of CO and H2 (100–300 atm) and temperatures greater than
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-019-00350-2) contains
supplementary material, which is available to authorized users.
* Banothile C. E. Makhubela
bmakhubela@uj.ac.za
1
Department of Chemistry, University of Johannesburg, P.O.
Box 524, Auckland Park 2006, South Africa
Vol.:(0123456789)
1 3

Transition Metal Chemistry
150 °C [10]. Using rhodium catalysts generally also result
in mild reaction conditions, such as pressures of 15–25 bar,
temperatures of 80–120 °C and reduced reaction time. In
addition, phosphine-modifed rhodium catalysts convert alk-
enes to linear and branched aldehydes with ratios as high
as 15–1.
of the reagents and substrates is controlled by unique steric
and electronic efects afecting the catalytically active sites
thereby leading to selective transformation [19].
Synthesis and characterization of pyrazolyl ligands
and rhodium complexes
Pyrazoles have been shown to demonstrate a wide array of
biological activity [11] especially antipyretic and antimicro-
bial. They are also useful as herbicides and fungicides, and
some pyrazole derivatives can be used as extracting agents
of various metal ions [12]. In addition to biological activ-
ity, pyrazoles have been used as neutral nitrogen-donating
ligands or as anionic monodentate ligands by deprotonation
of the pyrazolyl N–H proton [13]. The pyrazolyl ring can be
easily functionalized so as to construct multimetal coordina-
tion systems and to change the steric and electronic efects
for hydroformylation. As ancillary ligands, rhodium–pyra-
zolyl complexes have demonstrated high catalytic activity
in various catalytic processes such as the hydroaminometh-
ylation as well as the hydroformylation of 1-octene (Fig. 1)
[14]. Pyrazolyl ligands have also been found to facilitate
reduced induction periods in hydroformylation thereby [15]
resulting in remarkable catalytic activity. Hydroformylation
has also been carried out using supercritical carbon dioxide
(scCO2) [16]. Similar observations have been made with
other nitrogen-containing bis(phosphoramidite) ligands [17].
The main focus of this research was to convert 1-octene to
aldehydes by the use of a rhodium-based catalyst with pyra-
zolyl ligands. These catalysts were expected to have high
selectivity to aldehydes as a result of ligand structure and
substituents. With the use of pyrazolyl–pyridyl ligands in
rhodium pincer complexes, the synthesized ligands could act
as weak donors resulting in the metal centre having greater
electrophilicity as well as stabilizing various intermedi-
ates due to the rigid ligand structure. This will in turn aid
the coordination of the olefn during the hydroformylation,
leading to increased production of aldehydes [18]. Simi-
larly, with the choice of pyrazolyl ligands, the orientation
Materials and instrumentation
All reactions were carried out in air unless otherwise
stated. All solvents used were reagent grade, purchased
from Sigma-Aldrich and dried under nitrogen before use.
2,6-Bis(chloromethyl)pyridine, potassium hydroxide, sil-
ver trifate, hydrazine and phenyl diketone were purchased
from Sigma-Aldrich and used without further purifcation.
Spectrometer chemical shifts were reported relative to the
internal standard tetramethylsilane (δ 0.00 ppm) and ref-
erenced to the residual proton and carbon signals at 7.24
and 77.0 ppm, respectively, of CDCl3. Infrared spectra were
obtained neatly using a PerkinElmer Spectrum BX II ft-
ted with an ATR probe. Melting points were obtained using
a Gallenkamp Digital Melting point Apparatus 5A 6797.
Elemental analysis was performed on a Thermo Scientifc
FLASH 2000 CHNS-O Analyser. Mass spectrometry was
performed using Waters Synapt G2 mass spectrometer with
both ESI positive and Cone Voltage 15 V. XRD spectra were
obtained from a Bruker APEX-II CCD difractometer.
General synthesis of the ligands N′N′N pincer
ligands
A mixture of one equivalent 2,6-bis(chloromethyl)pyri-
dine and two equivalent of appropriate pyrazole in benzene
(40 mL), 40% aqueous KOH (12 mL) and 40% aqueous
tetrabutylammonium chloride (10 drops) was refuxed for
18 h. The organic layer was then separated, and evaporated
in vacuo. The crude product was washed with water to aford
an analytically pure compound as a white solid.
2,6‑Bis((3,5‑bis(trifuoromethyl)‑1H‑pyrazol‑1‑yl)methyl)
pyridine (1)
BF4
BF4
N
N
N
N
N
N
(Yield = 62%). Solubility: soluble in chloroform, hex-
N
N
Rh
N
Rh
ane, ethyl acetate; melting point 92–96 °C. FT-IR (vmax
/
OC
CO
PPh3
OC
1
cm−1): 1739.82(N=C); H NMR (400 Hz, CDCl3, 30 °C)
(ppm) = 5.54 (s, 4H, CH2); 6.89 (s, 2H, pz); 6.96 (d, 2H,
JHH =8.0 Hz py); 7.65 (t, 1H, py). 13C{1H} NMR (100 MHz,
CDCl3, 30 °C, δ, ppm); 56.53; 106.35; 120.30; 120.56;
134.43; 142.15; 143.11, 154.21; 158.11; Elemental analysis,
Anal. calcd. for C17H9F12N5: C, 39.94; H, 1.77, N, 13.70%;
Found C, 39.88; H 1.68; 13.99 N %.
F
3C
Ph
N
N
N
N
H
N
H
Fig. 1 Reported pyrazolyl ligands and rhodium–pyrazolyl complexes
used for hydroformylation [14, 16]
1 3

Transition Metal Chemistry
2,6‑Bis((3,5‑diphenyl‑1H‑pyrazol‑1‑yl)methyl)pyridine (2)
Yield = 87.2% Solubility: soluble in chloroform, hexane,
137.12; 139.75; 148.09; 149.23; 157.41 Elemental analysis,
Anal. calcd. for C11H13N3: C, 70.56; H, 7.00, N, 22.44%;
Found C, 70.11; H 6.61; 22.01 N %.
benzene, toluene; melting point 180–184 °C. FT-IR (vmax
/
1
cm−1): 1735.00(N=C); H NMR (400 Hz, CDCl3, 30 °C)
(ppm) = 6.81 (s, 2H, pz); 7.16 (s, 4H, py); 7.49 (m, 17H,
Ph, pz); 7.74 (t, JHH =7.8 Hz, 1H, py); 7.92 (m, 5H, Ph, pz).
13C{1H} NMR (100 MHz, CDCl3, 30 °C, δ, ppm); 54.77;
103.69; 119.95; 128.77; 129.21; 130.11; 131.21; 131.26;
135.21; 135. 77; 146.08; 151.45; 157.30. Elemental analy-
sis, Anal. calcd. for C37H29N5: C, 81.74; H, 5.38, N, 12.88%;
Found C, 81.39; H 5.68; 12.99 N %.
General synthesis of complexes
Two equivalents of the appropriate ligand in acetonitrile
(20 mL) was added to a stirring solution of one equivalent
of [Rh(CO)2Cl2] in a round-bottom fask. The yellow solu-
tion was stirred for 30 min at room temperature and then two
equivalence of silver trifate or sodium hexafuorophosphate
was added and the mixture was stirred for 24 h at room temp.
The product was fltered, and the fltrate was dried in vacuo
to yield yellow solids.
2,6‑Bis((3,5‑dimethyl‑1H‑pyrazol‑1‑yl)methyl)pyridine (3)
Yield=96% Solubility: soluble in chloroform, hexane, ben-
zene, toluene; melting point 120–125 °C. FT-IR (vmax/cm−1):
1750(N=C); 1H NMR (400 Hz, CDCl3, 30 °C) (ppm)=2.05
(s, 6H, CH3, pz); 2.21 (s, 6H, CH3, pz); 5.27 (s, 4H, CH2);
5.84 (s, 2H, pz); 6.58 (m, 2H, py); 7.46 (t, JHH = 7.8 Hz,
1H, py). 13C{1H} NMR (100 MHz, CDCl3, 30 °C, δ, ppm);
11.06; 13.50; 54.20; 105.68; 119.54; 138.15; 148.10; 157.04.
Elemental analysis, Anal. calcd. for C17H21N5: C, 69.12; H,
7.17, N, 23.71%; Found C, 69.55; H 7.00; 23.99 N %.
Synthesis of complex (6)
Yield=52%. Solubility: soluble in chloroform, hexane, ben-
zene; melting point-192 °C. FT-IR (vmax/cm−1): 1606.73
1
(C=N), 2000.22 (C=O) H NMR (400 Hz, d-acetone,
30 °C) (ppm): 6.02(d, 2H, JHH =15.6 Hz, CH2); 6.36(d, 2H,
JHH =15.2 Hz, CH2); 6.88 (s, 2H, pz); 7.45 (s, 6H, Ph, pz);
7.65 (s, 10H, Ph, pz); 8.05 (d, 2H, JHH =8.0 Hz, py); 8.18
(s, 4H, Ph, pz); 8.32 (t, 1H, py). 13C{1H} NMR (100 MHz,
CDCl3, 30 °C, δ, ppm); 54.77; 103.69; 119.95; 128.77;
146.08; 151.45; 157.30. Elemental analysis, Anal. calcd.
for C38H29FN5O2Rh: C, 64.05; H, 4.53, N, 9.83%; Found C,
64.21; H 4.77; 9.61 N %.
2‑((3,5‑Dimethyl‑1H‑pyrazol‑1‑yl)methyl)pyridine (4)
Yield = 87%. Solubility: soluble in chloroform, hexane,
benzene, toluene; melting point (liquid). FT-IR (vmax
/
1
cm−1): 1738.86(N=C); H NMR (400 Hz, CDCl3, 30 °C)
(ppm) = 5.32 (s, 4H, CH2); 6.15 (t, 2H, pz); 7.18 (d, 2H,
JHH = 8.0 Hz, py); 7.24 (d, 2H, JHH = 7.8 Hz, py); 7.65 (d,
2H, JHH =6.0 Hz, py); 8.49 (t, 1H, JHH =7.6 Hz py). 13C{1H}
NMR (100 MHz, CDCl3, 30 °C, δ, ppm); 57.37; 106.26;
120.50; 130.01; 138.11; 139.98; 156.56. Elemental analysis,
Anal. calcd. for C13H13N5: C, 65.25; H, 5.48, N, 29.27%;
Found C, 65.50; H 5.68; 29.50 N %.
Synthesis of complex (7)
Yield = 69%. Solubility: soluble in chloroform, DMSO;
Decomposition-205 °C. FT-IR (vmax/cm−1): 1556.58(C=N),
2003.11(C=O) 1H NMR (400 Hz, CD3CN, 30 °C)
(ppm): 1.96 (s, 6H, CH3); 2.45 (s, 6H, CH3); 4.57(d, 2H,
JHH =20.0 Hz, CH2); 5.67(d, 2H, JHH =20.0 Hz, CH2); 6.24
(s, 2H, pz); 8.15 (d, 2H, py); 8.48 (t, 1H, py). 13C{1H} NMR
(100 MHz, CDCl3, 30 °C, δ, ppm); 13.22; 13.59; 54.29;
104.61; 119.59; 138.15; 139.81; 142.02; 160.27; 191.27;
Elemental analysis, Anal. calcd. for C18H21TFN5O2Rh:
C46.56; H, 5.21, N, 15.08%; Found C, 46.13; H 5.66;
15.38 N %.
Synthesis of N′N bidentate ligand (5)
In a round-bottom fask, 1 eq of 2-chloromethyl pyridine and
1 eq of 3.5-bis(methyl)pyrazole were added with benzene
as solvent, 40% KOH (10 mL) as a base and 40% aqueous
tetrabutylammonium chloride (10 drops), the mixture was
then refuxed overnight. The organic layer was separated
and washed with water. After drying, a yellowish oil was
obtained.
Synthesis of complex (8)
Yield=86%. Solubility: soluble in chloroform, decomposi-
tion-170 °C. FT-IR (vmax/cm−1): 11,740.49 (C=N), 1996.36
1H NMR (400 Hz, CD3CN, 30 °C) (ppm): 5.32 (s, 4H, CH2);
6.53 (d, 2H, JHH =18.0 Hz, pz); 7.02 (m, 4H, pz); 7.64 (d,
2H, JHH = 8.0 Hz, py); 8.05 (m, 1H, pz). 13C{1H} NMR
(100 MHz, CDCl3, 30 °C, δ, ppm); 57.37; 106.26; 120.50;
130.01; 138.11; 139.98; 156.56; 191.2; Elemental analysis,
Yield = 54% Solubility: soluble in chloroform, hex-
ane, benzene; melting point (oil). FT-IR (vmax/cm−1):
1739.9(N=C) 1H NMR (400 Hz, CDCl3, 30 °C) (ppm)=2.19
(s, 6H, CH2), 5.32 (s, 2H, CH2); 6.22 (s, 1H, pz); 7.02 (m,
2H, py); 7.80 (m, 2H, py). 13C{1H} NMR (100 MHz, CDCl3,
30 °C, δ, ppm); 11.06; 13.52; 54.34; 105.72; 120.98; 122.40;
1 3

Transition Metal Chemistry
Anal. calcd. for C14H13F6N5OPRh: C, 32.64; H, 2.54, N,
13.59%; Found C, 32.13; H 2.68; 13.99 N %.
Results and discussion
Synthesis and characterization of pyrazolyl ligands
and corresponding complexes (1–9)
Synthesis of complex (9)
The ligands were synthesized by reacting one equivalent of
2,6-bis(chloromethyl)pyridine with one or two equivalent
of the appropriate pyrazole in benzene using excess KOH
(in order to deprotonate the pyrazolyl N–H hydrogen) and
tert-butylammonium chloride as a phase transfer agent. The
organic layer was then separated and evaporated in vacuo
with subsequent washing of the crude product with water
yielding the product. The ligands were generally obtained as
oils in good yields and are soluble in most organic solvents.
1H NMR characterization generally served to confrm forma-
tion of the ligands with methylene CH2 linker protons shift-
ing from around 4.9 ppm in 2,6-bis(chloromethyl)pyridine
to around 5.5 ppm for the ligands.
Two equivalent of compound 5 in acetonitrile (20 mL)
was added to a stirring solution of one equivalent
of [RhCl(COD)2] in a round-bottom fask. The yellow solu-
tion was stirred for 30 min at room temperature and then
two equivalence of silver trifate was added and the mix-
ture stirred for 24 h at room temperature. The product was
fltered, and the fltrate was dried in vacuo to yield yellow
solids.
Complex (9)
Yield = 66%, Solubility: soluble in chloroform; melting
point 149–150 °C. FT-IR (vmax/cm−1): 1732.00(N=C),
These ligands readily formed complexes with rhodium
upon reaction with the appropriate rhodium precursor using
AgOTF or NaPF6 as halide abstractors (Scheme 1). The
resulting complexes were obtained in good yields as yellow
solids and are stable in air at room temperature for extended
periods of time (ca 12 months). In addition, these complexes
are generally soluble in solvents such as dichloromenthane
and chloroform.
1
2113.55(C=O); H NMR (400 Hz, CDCl3, 30 °C) (ppm):
1.96 (12H, s, CH3); 2.77 (6H, s, CH3, pz); 5.32 (2H, s,
CH2); 5.5 (2H, s, CH); 6.15 (1H, s, pz); 7.18–8.67 (4H,
m, py). 13C{1H} NMR (100 MHz, CDCl3, 30 °C, δ, ppm);
11.11; 12.24; 18.98; 31.21; 59.87; 103.29; 120.50; 121.22;
130.01; 138.11; 139.98; 156.56; 156.92; 161.20; Elemen-
tal analysis, Anal. calcd. for C21H31TFN3ORh: C, 54.08; H,
7.35, N, 9.01%; Found C, 54.49; H 7.55; 8.98 N %. HR-
ESI–MS[M]+: 428.1566.
1
Generally, in the H NMR spectra of the complexes’,
downfeld shifts in the positions of the pyrazolyl protons
served as an indication of successful complexation of the
6-/OTf
+
Scheme 1 Synthesis of pyra-
PF
R
R
zolyl–rhodium complexes
N
N
N
Rh
N
R
N
N
N
N
CO
CO
0.5eq
]
)
2 2
[RhCl(
or
R
N
rt 24h
NaPF6
R
AgOTf
N
R
R
R
=
=
=
R
R
R
6
Ph;
7
Me,
8
H,
-OTf
+
N
N
COD
N
]
[RhCl(
)
Rh
2
N
N
N
rt 24h
AgOTf,
9
1 3

Transition Metal Chemistry
ligands to rhodium. In addition, the methylene CH2 linker
from both nitrogen atoms into the pyrazolyl ring and hence a
stronger electron-donating efect. The bond lengths between
rhodium and the ancillary ligands as well as the various bond
angles are in conformity with other N′N′N rhodium–pyra-
zolyl complexes reported in the literature [21]. The crystal
data and structure refnement parameters for this complex
are given in Table 1.
1
protons which show as a singlet in the H NMR of the
ligands split into two distinct doublets upon complexation.
Such splitting was observed because the linkers were now
in diferent environments, that is, in axial and equatorial
positions of a six-member ring upon complexation. Simi-
lar observations have been reported in the literature [20].
In addition, mass spectrometry confrmed the formation of
complex 9 as the m/z of the cationic half of the complex
was observed.
Catalytic activities of the complexes 6–9
for the hydroformylation of 1‑octene
Molecular structure of 8
Catalysts 6–9 were evaluated for the hydroformylation reac-
tions by optimizing a variety of reaction conditions such as
the syngas (CO and H2, 1:1 ratio) pressure, reaction time,
reaction temperature and catalyst loading using 1-octene and
n-decane as an internal standard. In a typical run, to 5 mL
of toluene and 0.25 mL of n-decane, the required amount of
catalyst and alkene were added, and the reactors were pres-
surized with syngas (CO and H2, 1:1 ratio) at the required
pressure. The reaction was then left to react for the specifed
Crystals suitable for X-ray difraction experiments for com-
plex 8 were obtained by slow evaporation of a DCM solution
of 8. The crystal structure for 8 confrmed the coordination
of the ligand to the rhodium centre with the geometry around
the rhodium centre being square planar (Fig. 2).
Complexes 8 crystallize in the orthorhombic crystal sys-
tem with space group Pna21 with a slightly distorted square
planar geometry around the rhodium centre. The N3-Rh1-
N4 bond angle is 173.8° instead of the expected 180° for a
truly square planar complex. This slight deviation may be
as a result of the rigidity of the resulting complex conferred
by coordination of the three N atoms of the pincer system,
resulting in increased stability. In addition, it is interesting
to note that the rhodium pyridinyl nitrogen bond is much
longer than the rhodium–pyrazolyl bonds. This observa-
tion indicates that coordination from the pyrazolyl nitrogen
atoms is stronger as a result of the delocalization of electrons
Table 1 Crystal data and structure refnement for 8
Identifcation code
Empirical formula
Formula weight
8
C14H13F6N5OPRh
38.16
Temperature/K
Crystal system
Space group
100 K
orthorhombic
Pna21
a/Å
b/Å
c/Å
α/°
10.0373(8)
20.8699(16)
8.6902(6)
90
β/°
γ/°
90
90
1820.4(2)
54
1.8795
Volume/Å3
Z
ρcalcg/cm3
μ/mm−1
1.101
F(000)
1012.6
Crystal size/mm3
Radiation
0.3×0.2×0.2
Mo Kα (λ=0.71073)
3.9–52.82
2θ range for data collection/°
Index ranges
−12≤h≤12, −26≤k≤22,
−7≤l≤10
Refections collected
12,580
Independent refections
Data/restraints/parameters
Goodness-of-ft on F2
Final R indexes [I≥2σ (I)]
Final R indexes (all data)
Largest dif. peak/hole/e Å−3
Flack parameter
3370 [Rint =0.0561, Rsigma =0.0547]
3370/1/253
1.040
R1 =0.0358, wR2 =0.0713
R1 =0.0504, wR2 =0.0774
0.46/−0.61
Fig. 2 Molecular structure of 8; Selected bond lengths; Rh1-
N1: 2.117(4), Rh1-N3: 2.036(4), Rh1-N4: 2.037(4), N3-Rh1-N4:
173.81(15)°, N1-Rh1-N4: 86.86(14)°
0.05(5)
1 3

Transition Metal Chemistry
reaction time and temperature. Samples were then analysed
before starting the hydroformylation reaction and also at
the end of the hydroformylation reaction using gas chro-
matography (GC) in order to determine alkene to aldehyde
conversion and product selectivity (Fig. SI-1). In the case of
undecene and dodecene, characterization of reaction con-
tents was achieved by 1H NMR.
is possibly as a result of catalyst decomposition under ele-
vated syngas pressures. Taking into account both 1-octene
conversions, selectivity to aldehyde and n/iso ratios of alde-
hyde for 6–8 at the diferent pressures, optimum pressures
chosen for the hydroformylation reaction for 6, 7 and 8 were
40, 20 and 30 bar, respectively (Table SI-II).
Generally, reducing the catalyst loading from 0.08 to
0.04 mol% for 6 and 7 resulted in only marginal decreases
in conversion from around 100 to around 98%, while in the
case of 8, there was a signifcant drop in conversion from
around 94 to 50%. However, in the case of 6 and 7, reducing
the catalyst loading to 0.02 mol% from 0.04 mol% resulted
in a signifcant decrease in conversion from around 98 to
42% and 16% for 6 and 7, respectively. In addition, when
catalyst loading is reduced, there was also an observed cor-
responding reduction in aldehyde selectivity (Table SI-III).
Generally, at 16 h, there is a quantitaive conversion of
1-octene to aldehydes as observed using the optimum tem-
perature, pressure and amount of catalyst for each of the
pre-catalysts 6–8 for the hydroformylation reaction. Reduc-
ing the reaction time from 16 h to 12 h for 6 and 7 did not
signifcantly afect conversions; however, a further reduction
in the reaction time to 6 h resulted in a signifcant drop in
conversion from around 95 to around 20%. In the case of 8,
there was a signifcant drop in conversion from around 94 to
58% by reducing the reaction time from 16 h to 12 h. This
indicated the ideal reaction time to be 16 h. The optimized
reaction conditions for the hydroformylation of 1-octene for
pre-catalysts 6–8 are recorded in Table 2.
Optimization of reaction conditions
Preliminary hydroformylation reactions were conducted
using pre-catalysts 6–8 at 75 °C and 90 °C using 1-octene
as substrate at 30 bar. In the case of 6, it is noticed from
temperature optimization (Table SI-I) that the conversion at
75 °C was 97% with mainly branched aldehyde production,
while increasing the temperature to 90 °C (conversion 46%)
mainly resulted in isomerization. In the case of both 7 and
8, increasing the temperature from 75 to 90 °C resulted in a
decrease in activity possibly as a result of catalyst decom-
position at higher temperature. Comparing the catalyst to
each other at these two temperature, it is seen that at 75 °C,
and catalyst 8 is the most active compared to the other two
with a TON of 1584. The activity trend at 75 °C is 8>7>6.
In addition, the ratios of aldehydes indicates 8 forms more
linear aldehydes than the other two. The bulk around 6 and 7
may reduce substrate interaction with the metal centre which
makes it less selective to linear aldehydes.
Reducing the syngas pressure from 30 bar to 20 bar for
6 results in isomerization as well as a drop in conversion
of around 10%. Increasing the pressure to 40 bar resulted
in an increase in conversion to 100% with a corresponding
increase in selectivity to aldehydes. Similarly for 7 and 8,
reducing the pressure to 20 bar resulted in a marked decrease
in conversion to around 10% for 7; however, for 8, no con-
version was observed. However, increasing the pressure to
40 bar resulted in a drop in conversion to around 60% for
7 and 44% for 8 with mostly isomerization products being
formed (Table SI-II). This drop in conversion and selectivity
Pre-catalyst 9 was then evaluated for the hydroformyla-
tion of 1-octene under the optimum reaction conditions of
7 since both pre-catalysts share similar ligand structure
(Table 2). Under similar reaction conditions for 7 and 9,
pincer catalyst 7 demonstrated superior performance than
9 for the hydroformylation reaction. There is a signifcantly
reduced conversion when the ligand systems are changed
from a pincer ligand to a less rigid N′N bidentate system.
In addition, the amount of aldehyde formed signifcantly
Table 2 Optimization of
Entry
Catalyst
Time
Pressure (bar)
Conver- % iso octene
sion (%)
% aldehyde
n:iso
TON
reaction time and efect
of ligand for 1-octene
hydroformylation
1
2
3
4
5
6
7
8
9
6
7
8
6
7
8
6
7
9
16
16
16
12
12
12
6
40
20
30
40
20
30
40
20
20
99
99
94
99
98
58
29
18
68
8
4
24
16
28
92
96
77
84
72
20
0
0.9
0.9
1.8
0.93
1.26
2.38
0
2033
2109
1584
1835
1567
1625
0
80
100
100
98
6
16
0
2.12
0
0
0
32
Conditions 75 °C, catalyst loading: cat loading, 6, 7 and 9 (0.04 mol%), 8 (0.08 mol%), toluene 5 mL,
0.25 mL n-decane. TON (mmol product/mmol Rh)
1 3

Transition Metal Chemistry
drops in the case of using the N′N bidentate system, yield-
ing only 2% aldehydes. This poor activity of 9 as com-
pared to 7 could be as a result of the increased stability or
rigidity of 7 imposed by the pincer ligand. However, the
presence of COD ligand in 9 which is absent in 7 may also
play a role in the formation of the catalytically active Rh-H
species leading to diferent catalytic activities.
dodecene in the presence of 7 resulted in a drop in conver-
sion of the alkene to the aldehyde (Table 3). In this case,
isomerization seems to be more prevalent under these reac-
tion conditions. This does not, however, preclude the cata-
lyst from hydroformylating these substrates under diferent
reaction conditions in which aldehyde formation may be
observed.
Over the course of optimization when taking into account
the conversion, product selectivity and product distribution,
it is evident that pre-catalyst 7 is the best performing cata-
lyst. This is interesting considering that 7 possesses methyl
substituents on its ligand, which is less bulky than pre-cat-
alyst 6 (which has phenyl substituents on its ligand). This
interesting observation on catalytic activity could possibly
be due to a combination of both steric and electronic efects.
In terms of steric efects, a plausible explanation for reduced
activity for more bulky groups could be hindering substrates
from the metal centre. However, some amount of electron
donation may also play a role in the rate of hydroformyla-
tion. Pre-catalyst 6, which has phenyl substituents on the
pyrazole ring, may behave as electron-withdrawing groups,
resulting in electron density being pulled away from the
metal centre thereby reducing the rate of hydroformylation
[22]. This infuence of electron donation then results in 7
with electron-donating methyl substituents on the pyrazolyl
ligand being more active than both 6 and 8. Complexes 6–9
were found to be very active when compared to other rho-
dium–pyrazole containing systems. On average, complexes
6–9 required lower amounts of syngas pressure (around
30 bar compared to 50 bar), slightly lower temperature
(around 75 °C compared to 80 °C), the absence of extra sup-
porting ligand or the use of expensive scCO2 as a solvent,
albeit with longer reaction times (around 16 h compared
to 5 h) [14–16]. When [Rh(CO)2Cl2] and [RhCl(COD)]2
were used for hydroformylation under the optimum reaction
conditions established for 7, the importance of the ligand
structure was evident as there was a signifcant drop in cata-
lytic and selectivity (Table 3).
Conclusions and future outlook
An N′N pyrazolyl complex and a series of N′N′N pincer
pyrazolyl complexes have been successfully synthesized,
characterized and evaluated for their catalytic activity in the
hydroformylation reaction. All the catalysts demonstrated
activity in the hydroformylation reaction, and the best per-
forming catalyst was 7 at 20 bar syngas pressure, 75° C, over
16 h. Under these reaction conditions, 99% conversion of
1-octene can be achieved with 96% selectivity for aldehydes.
This activity is possibly a result of the combination of both
steric and electronic efects. Bulky and electron-withdraw-
ing substituents were less favoured for the hydroformylation
reaction in comparison with less bulky but electron-donat-
ing methyl substituents. In addition, pincer complexes 6–8
were observed to be better suited for the hydroformylation
reaction as compared to N′N bidentate complex 9. Hydro-
formylation of other olefnic substrates, such as undecene
and dodecene, has been investigated, but these reactions
were less selective towards aldehydes—efectively showing
that these catalysts tend to favour isomerization with longer
chain alkenes.
Supplementary data
Electronic supporting information (ESI) is available free of
charge. CCDC 1920507 contains the supplementary crystal-
lographic data for complexes 8. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retri
eving.html or from the Cambridge Crystallographic Data
Under optimized reaction conditions for hydroformyla-
tion of 1-octene, varying the alkene to either undecene or
Table 3 Variation of substrates
Entry
Catalyst
Pressure (bar)
Substrate
Conversion % aldehyde
(%)
n:iso
for the hydroformylation
reaction
1
2
3
4
5
7
20
20
20
20
20
1-octene
Undecene
Dodecene
1-octene
1-octene
99
70
90
35
29
96
0.3
0.31
0
0.16
1.64
0.18
0
7
7
[Rh(CO)2Cl2]
[RhCl(COD)]2
54
0.63
Conditions 75 °C, catalyst loading: (0.04 mol%), time 16 h, toluene 5 mL, 0.25 mL n-decane. TON (mmol
product/mmol Rh). Conv determined by 1H NMR
1 3

Transition Metal Chemistry
7. Le L, Couturier J, Dubois J (2016) J Mol Catal A 417:116–121
8. Sun Q, Dai Z, Liu X, Sheng N, Deng F, Meng X, Xiao F, Sun Q,
Dai Z, Liu X, Sheng N, Deng F, Meng X (2015) J Am Chem Soc.
https://doi.org/10.1021/jacs.5b02122
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements We acknowledge the National Research Founda-
tion of South Africa (NRF) (Grant Nos. 105557, 117989), The Tech-
nology and Human Resource for Industry Programme (THRIP), (Grant
No. THRIP/58/30/11/2017), Sasol SA Ltd (University Collaboration
Programme) and the University of Johannesburg’s Centre for Synthesis
and Catalysis for fnancial support.
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Compliance with ethical standards
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Organometallics 22:5261–5267
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