The effects of metals and ligands on the oxidation of n-octane using iridium and rhodium “PNP” aminodiphosphine complexes

Article abstract of DOI:10.1177/1747519820967245

Ir and Rh “PNP” complexes with different ligands are utilized for the oxidation of n-octane. Based on the obtained conversion, selectivity, and the characterized recovered catalysts, it is found that the combination of Ir and the studied ligands does not promote the redox mechanism that is known to result in selective formation of oxo and peroxo compounds [desired species for C(1) activation]. Instead, they support a deeper oxidation mechanism, and thus higher selectivity for ketones and acids is obtained. In contrast, these ligands seem to tune the electron density around the Rh (in the Rh-PNP complexes), and thus result in a higher n-octane conversion and improved selectivity for the C(1) activated products, with minimized deeper oxidation, in comparison to Ir-PNP catalysts.

Full text of DOI:10.1177/1747519820967245

967245CHL0010.1177/1747519820967245Journal of Chemical ResearchNaicker et al.
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Journal of Chemical Research
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The effects of metals and ligands on the
oxidation of n-octane using iridium and
rhodium “PNP” aminodiphosphine complexes
https://doi.org/10.1177/1747519820967245
DOI: 10.1177/1747519820967245
journals.sagepub.com/home/chl
Dunesha Naicker, Saba Alapour and Holger B Friedrich
Abstract
Ir and Rh “PNP” complexes with different ligands are utilized for the oxidation of n-octane. Based on the obtained
conversion, selectivity, and the characterized recovered catalysts, it is found that the combination of Ir and the studied
ligands does not promote the redox mechanism that is known to result in selective formation of oxo and peroxo
compounds [desired species for C(1) activation]. Instead, they support a deeper oxidation mechanism, and thus higher
selectivity for ketones and acids is obtained. In contrast, these ligands seem to tune the electron density around the
Rh (in the Rh-PNP complexes), and thus result in a higher n-octane conversion and improved selectivity for the C(1)
activated products, with minimized deeper oxidation, in comparison to Ir-PNP catalysts.
Keywords
Aminodiphosphine, iridium, n-octane, oxidation, rhodium
Date received: 31 July 2020; accepted: 29 September 2020
over sp3 hybridized C–H bonds.12–14 Furthermore, the
chemical inertness of alkanes, together with their high ioni-
Introduction
The activation of alkanes is a desirable but challenging
zation energy, pKa values, and low electron affinity, makes
reaction due to the saturated carbon–hydrogen bonds hav-
activation of C–H bonds difficult.11,13,15–17
ing relatively inert character. In addition, the activated
To overcome such problems, there has been a rapid devel-
products are often more reactive than the alkane substrate,
opment of catalytic systems inspired by the biological sys-
which may lead to over-oxidation of the products. There is
tems of cytochrome P450 and methane monooxygenase.18–27
a current need and economic urgency to replace current pet-
rochemical feedstocks (olefins) with easily accessible
alkanes, which can result in more proficient use of energy
Catalysis Research Group, School of Chemistry and Physics, University
of KwaZulu-Natal, Durban, South Africa
and efficient strategies for fine chemical synthesis.1–10
Terminally oxidized hydrocarbons are valuable starting
Corresponding author:
Holger B Friedrich, Catalysis Research Group, School of Chemistry and
Physics, University of KwaZulu-Natal, Westville Campus, Durban 4000,
South Africa.
materials in the pharmaceutical and chemical industries.11
However, one of the main challenges in the activation of
alkane C–H bonds is the low selectivity in forming the
desired products, that is, the preferential activation of sp2
Email: friedric@ukzn.ac.za
Creative Commons Non Commercial CC BY-NC: This article is distributed under the terms of the Creative Commons
Attribution-NonCommercial 4.0 License (https://creativecommons.org/licenses/by-nc/4.0/) which permits non-commercial use,
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2
Journal of Chemical Research 00(0)
Figure 1. General reaction scheme and the structures of the complexes used as catalysts for the oxidation of n-octane.74
These enzymes have the ability to efficiently catalyze a num- the oxidant was carried out. The substituents on the nitro-
ber of exogenous and endogenous organic compounds, such gen atom were varied by making use of six functional
as medium chain alkanes, to large molecules including groups (Figure 1), resulting in different properties such as
triterpenes, as well as steroidal, polyaromatic compounds, basicity, steric hindrance, and various bite angles to see
and, more importantly, oxygenates.28–36 Site selective C–H their effects on the catalytic performance and the reaction
activation by iron complexes has been reported by White and mechanisms of these catalysts during n-octane oxidation.
co-workers in trying to mimic enzymes.37–41 These catalytic Recycling studies and characterization of the used catalysts
processes are carried out by a variety of oxidants, namely, were performed, which provided some insights regarding
PhIO, NaOCl, H2O2, alkyl hydroperoxides, percarboxylic the mechanism behind the relatively stable catalyst perfor-
acids, and molecular oxygen.29,34,42–49
mances through three reaction cycles.
The design of a suitable ligand system is one way of
achieving selective activation of paraffins. These systems
could include the aminodiphosphine or PNP ligand
Catalyst preparation
system.50–58 These hybrid ligands, which consist of both The complexes 1 and 2 (Figure 1) were synthesized by the
soft (N) and hard (P) donors, are of particular interest in procedure reported elsewhere.74 The PNP ligands were
that they are part of a system that can display high activity, added to methanol/acetone mixtures (2:1; v/v), to which the
stability, and variability.57–62 By modifying the ligand metal complex precursor and NH4PF6 salt were added. The
backbone through substitution of the donor atoms, the complexes were characterized by NMR and infrared (IR)
activity of the metal can be tailored.63,64 These ligands may spectroscopy.
bind to the metal centers as either monodentate, bidentate,
or as bridged ligands, which allow the reactions of the
metal ions to be selective due to the high demand the Results and discussion
ligands place on the stereochemistry of the complex.65
Oxidation of n-octane
In general, phosphine-based ligands are limited in their
application in oxidation reactions due to ligand loss and The optimization of the substrate to oxidant ratio. Our recent
degradation.66 Ruthenium-based phosphine complexes study on the application of these catalysts showed that these
have been studied for the oxidation of n-octane; however, two families of complexes follow similar chemistry for sty-
low conversions and over-oxidation are prevalent.67,68 The rene oxidation.74 Therefore, 1a was only chosen for the ini-
activation of alkane C–H bonds by rhodium and iridium tial optimization studies. The reaction was initially carried
complexes has not been thoroughly explored. However, out at 80°C with tert-butyl hydroperoxide (TBHP) as the
Nomura and Uemura69 have used rhodium compounds such oxidant in acetonitrile using catalyst 1a (Figure 2). The
as Rh3O, [Rh(acac)3], [RhCl(CO)2]2, [RhCl(PPh3)3], and catalyst/substrate molar ratio was maintained at 1:100, and
[Rh2(OAc)4] for the oxidation of cyclohexane using per- then the substrate/oxidant molar ratio was varied (1:2.5;
acetic acid, H2O2, TBHP (t-butyl hydroperoxide), and 1:5; 1:7.5; 1:10) (Figure 2). The ratio of 1:5 was found to be
m-CPBA (meta-chloroperoxybenzoic acid).69
the optimum considering both highest conversion and low-
Considering the worldwide increase in gas to liquid est selectivity for the ketones (the over-oxidation products).
plants, there is an increase in the production of medium- to The conversion at substrate/oxidant ratios greater than 1:5
long-chain paraffins, which increases the need to function- showed negligible increases, suggesting that the quantity of
alize these hydrocarbons.70–72 In continuation of our previ- the loaded catalyst to activate the oxo species was the limit-
ous work on metal complexes containing aminodiphosphine ing reagent at this substrate/oxidant ratio. Lowering the
ligands for n-octane oxidation,73 this study focuses on the catalyst/substrate ratio to 1:50 at a 1:5 ratio of substrate/
application of iridium (1) and rhodium (2) aminodiphos- oxidant ratio did not change the conversion, but resulted in
phine “PNP” complexes (Figure 1) for the oxidation of greater selectivity toward ketones that are of less interest in
n-octane. Optimization of the solvent, the temperature, and comparison to the aldehyde or alcohols.

Naicker et al.
3
9
8
7
6
5
4
3
2
1
0
Conversion (Ir)
Conversion (Rh)
Blank Catalyst
16
13
10
7
3
0
a
b
c
d
e
f
Catalyst
1/2.5
1/5.0
1/7.5
1/10.0
Substrate/Oxidant
Figure 4. Screening of the iridium (Ir) and rhodium (Rh)
catalysts in n-octane oxidation with the different substituents
on the nitrogen atom (a–f).
Figure 2. Optimization of the substrate/oxidant ratio in the
oxidation of n-octane.
Conditions: catalyst/substrate (1:100); temperature: 80°C; solvent: MeCN;
catalyst: 1a. In the case of the blank reaction, no catalyst was added.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; temperature: 80°C.
R=cyclohexyl (a); isopropyl (b); pentyl (c); phenyl (d); o-chlorophenyl
(e); m-methoxyphenyl (f); Cp*=η5-pentamethylcyclopentadienyl.
Blank
Catalyst 1a
11
9
the catalyst, and hence poor catalytic performances were
achieved. This finding is in agreement with reports that
some catalysts are not able to activate certain peroxides,
resulting in a low selectivity of the desired products.78–80
7
5
Metal. Using the optimum conditions (catalyst/substrate
molar ratio of 1:00, substrate/oxidant molar ratio of 1:5 at
80°C in DCE), screening of the iridium (1) and rhodium (2)
catalysts (Figure 1) bearing different substituents on the nitro-
gen atom of the ligand backbone was undertaken. Interest-
ingly, both the Rh and Ir–based catalysts showed relatively
high conversion of n-octane. In general, the Rh catalysts were
found to be more active than the corresponding Ir catalysts
with the same substituents (Figure 4). This may be due to the
rhodium complexes having a greater ability to coordinate to
larger hydrocarbons than the iridium catalysts.81 Periana and
Bergman have shown that the relative rate constants for rho-
dium catalysts are almost double those of the iridium catalysts
in reactions involving n-hexane and n-pentane.82
Comparing the catalysts with the alkyl substituents on
the nitrogen atom (a–c) (Figure 1), the longer alkyl chains
render the complexes more basic, as in catalysts 1c and 2c.
In theory, the more basic the nature of an organometallic
catalyst, the stronger is its electron-donating ability that
results in a greater electron density located on the metal
center. As discussed in a previous study, this renders it eas-
ier to go from the MI to MIII states (oxidation) upon activa-
tion by the oxidant. However, the formation of the active
tert-butoxy radical becomes more difficult, since the highly
basic nature of the complex stabilizes the MIII oxidation
state. It was noted that electron-donating substituents
decrease the reactivity of the oxo species.23 Catalysts 2a
and 2b are comparable in terms of their activity, which is
also observed with Cr complexes bearing the same ligand
backbone in ethylene oligomerization.51 The Rh catalyst
with the phenyl substituent (2d) gives the highest conver-
sion (13%). Comparing the catalysts with the substituted
phenyl rings, it was observed that the chloro (e) or the
methoxyphenyl (f) group has very little effect in changing
the catalytic activity (Figure 4).
2
0
25 °C
50 °C
Temperature/ °C
80 °C
Figure 3. Optimization of the temperature in the oxidation of
n-octane.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; catalyst: 1a. In the case of the blank reaction, no catalyst was added.
Temperature optimization. Before optimizing the temperature
for this reaction, another popular solvent [dichloroethane
(DCE)] was tested for this reaction at the same temperature as
for the optimized conditions in MeCN. Interestingly, a better
catalytic performance was observed that might be linked to
the better solubility of the substrates in this solvent in com-
parison to MeCN. Thereafter, the temperature optimization
was performed using DCE at 25, 50, and 80°C (Figure 3)
with a catalyst/substrate molar ratio of 1:100 and a substrate/
oxidant molar ratio of 1:5. At 25°C, octanoic acid was the
only product that formed with a 1% conversion of n-octane.
At 50°C, a 4% conversion of n-octane was noted. The highest
conversion (9%) was obtained at 80°C with good selectivity
for the primary activation products (C1 activation). This also
could be attributed to the solubility of the oxidant in the sol-
vent. Similar studies carried out in alkene oxidation showed
better conversion in chlorinated solvents.75,76 It has been
shown that some catalytic reactions proceed more efficiently
in a mixture of solvents;77 however, when a 1:1 ratio of DCE/
MeCN was used, the reactions with and without the catalyst
were comparable.
Oxidants. Hydrogen peroxide and m-CPBA (meta-chloro-
peroxybenzoic acid) were also investigated for this reac-
tion. However, the use of these two oxidants decomposed

4
Journal of Chemical Research 00(0)
2a
2b
2c
2d
2e
2f
1a
1b
1c
1d
1e
1f
100
80
60
40
20
0
100
80
60
40
20
0
Octanols
Octanones
Octanal
Octanoic acid
Octanols
Octanones
Octanal
Octanoic acid
Products
Products
Figure 6. Selectivity for the products of oxidation of n-octane
by the Rh (2) catalyst.
Figure 5. Selectivity for the products of oxidation of n-octane
by the Ir (1) catalysts.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; temperature: 80°C.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; temperature: 80°C.
M=Rh (2); R=cyclohexyl (a); iso-propyl (b); pentyl (c);
phenyl (d); o-chlorophenyl (e); m-methoxyphenyl (f); Cp*=η5-
pentamethylcyclopentadienyl.
M=Ir (1); R=cyclohexyl (a); isopropyl (b); pentyl (c); phenyl
(d); o-chlorophenyl (e); m-methoxyphenyl (f); Cp*=η5-
pentamethylcyclopentadienyl.
The selectivity for the products of oxidation by the Ir certain products, besides the already discussed role in
catalysts is shown in Figure 5. Deeper oxidation to the improving the catalyst basicity that was discussed in a pre-
ketonic product is prevalent, as seen by the high selectivity vious study using these catalysts.74
for the octanones (59%–77%) for all the Ir-based catalysts.
The deep oxidation process (ketone formation) is much
The extent of the oxidation is referred to as depth here. So, slower with the rhodium catalysts in comparison to the Ir
deep oxidation refers to the catalyzed reactions that yield catalysts (Figure 6), which is noted by the higher selectivity
highly oxidized products like the acid. Secondary or mod- for the octanols (11%–41%), with catalyst 2b being the
erate oxidation (deeper oxidation) is a relative term, mean- most selective (41%) and 2c being the least selective toward
ing if it is used in the case of alcohols, it refers to ketones these octanols. As also noted with the iridium catalysts, the
(deeper oxidation products). When applied to ketones, it C(1) position is the least active, with the C(2) and C(3)
refers to acids (deeper oxidation).
positions being the most active carbons (Table 1). The
However, the octanols are the second major products higher activity of internal carbons is also observed in other
with selectivities of 18%–26%. The C(1) position of the oxidation systems.15,43,83 Research has been quite limited
n-octane chain is the least activated, with the C(2) and (C3) using Ir and Rh catalysts for the oxidation of alkanes.
positions being the most activated carbons. For the cata- Nomura and Uemura reported a TON (turnover number) of
lysts with ligand backbones containing substituted phenyl 93 for the oxidation of n-octane using Rh catalysts.69 In
groups (1e and 1f), over-oxidation of 1-octanol to octanal, other reported studies, the regioselectivity parameters at the
and thereafter octanoic acid is much slower in comparison C(2), C(3), and C(4) positions of the alkane chain are much
to the catalysts with alkyl substituents. This might be due to higher, due to the primary selectivity being substantially
the conjugation effect of the phenyl ring via resonance that lower, whereas in this system, the primary selectivity, the
decreases the charge density around the metal center, and C(1) position, is high, accounting for the lower internal
therefore decreases the formation of over-oxidation prod- regioselectivity parameters.15,43,44,48,83,84 Unlike the system
ucts. For both catalysts 1e and 1f, a 3% selectivity for reported in this study, other studies report no selectivity for
1-octanol was observed with 7% and 5% selectivities for the products with an activated terminal carbon, with only
octanal, respectively. However, for the catalysts with alkyl the ketones being obtained.34,39,67,68 Therefore, Ir-based
substituents, 1a and 1b, 17% and 11% selectivities for catalysts followed the expected pathways and resulted in
octanols and octanoic acid are observed, with no selectivity the activation of internal carbons within octane, but Rh
to octanal (indicating that these ligands with Ir cannot per- with the studied ligands seems to switch the reaction from
form the mild activation of the terminal C of n-octane). the oxidation of the internal to terminal carbons, that are
This suggests that catalysts 1a and 1b are well capable of indeed more interesting for industry since C(1) activated
oxidizing the aldehydes to acids. In contrast, catalyst 1d, alkanes can be easily transformed into α-olefins that are in
that contains an unsubstituted phenyl ring in its structure, high demand by polymer industries.
did not result in the formation of 1-octanol, octanal, and
According to the recent work by Chepaikin et al.,85 oxida-
octanoic acid, which suggests that this catalyst cannot acti- tive homogeneous catalysis of alkanes can follow two mecha-
vate the n-octane at its α position [C(1)]. Thus, it seems that nisms. Mechanism A is referred to as an outer-sphere
substitution of the aromatic ring and changing the elec- mechanism (A), where the reaction takes place using the in
tronic structure of the complexes containing phenyl rings situ generated oxo and peroxo metal complexes. This reaction
easily modifies the reaction selectivity with these catalysts. mechanism is known to yield alcohols (Scheme 1). In con-
This clearly confirms the contribution of the nitrogen atoms trast, the inner-sphere mechanism (B) happens mainly with
of the ligands in these complexes in tuning the selectivity to catalysts that contain Cl with M-alkyl radical intermediate

Naicker et al.
5
Table 1. Selectivity parameters for the Ir and Rh catalysts in the oxidation of n-octane.a
Catalyst
Ir
Alcoholb,c C(1):C(2):C(3):C(4)
Ketoneb,c C(2):C(3):C(4)
Totald C(1):C(2):C(3):C(4)
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
1:9:12:9
1:5:6:4
1:4:5:3
0:1:2:1
1:4:0:4
1:3:3:4
1:4:5:4
0:1:1:1
1:2:1:2
1:1:6:4
1:2:4:2
1:3:6:5
1:1:1
1:1:1
1:1:1
1:2:2
1:1:1
1:1:1
1:1:1
1:1:2
1:2:1
1:1:2
1:1:1
1:1:2
1:2:2:2
1:3:3:3
1:17:16:16
0:1:2:2
1:2:1:2
1:2:2:2
1:2:2:3
1:2:2:3
1:2:3:3
1:3:5:5
1:1:2:2
1:1:1:2
Rh
aConditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent: DCE; temperature: 80°C.
bParameters C(1):C(2):C(3):C(4) are the relative reactivities of the hydrogen atoms at carbons 1, 2, 3, and 4 of the n-octane chain.
cThe calculated reactivities from the % selectivity are normalized, that is, calculated taking into account the number of hydrogen atoms at each
carbon.
dIt includes the % selectivity of octanoic acid, octanal, alcohols, and ketones and the values are normalized.
Scheme 1. Outer-sphere (A) and inner-sphere (B) and free-radical (C) mechanisms of C–H bond activation in alkanes.85,86
formation, and results in the deep oxidation of alkanes to important to note that in the majority of cases, the latter two
ketone and acids. Also, the free-radical mechanism (C) is mechanisms dominate. However, generally, and considering
known to result in deep oxidation and forms products such as the selectivity for different products observed over the
ketones and acids as discussed for mechanism (B).86 It is Rh-based catalysts (except for 2c), the data support the slight

6
Journal of Chemical Research 00(0)
Catalyst 1c
Catalyst 2c
Fresh
Cycle 1
Cycle 2
14
11
8
100
75
50
25
0
6
3
Octanols
Octanones
Octanal
Octanoic acid
0
Fresh
Cycle 1
Cycle 2
Products of oxidation
Figure 7. Recyclability testing of catalysts 1c and 2c in the
oxidation of n-octane.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; 80°C.
Figure 8. Recyclability testing of catalyst 1c: selectivity for the
products of oxidation.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; 80°C.
involvement of mechanism A during oxidation using these
catalysts, which is in agreement with the findings of Chepaikin
et al.85 In contrast, Ir-based catalysts very dominantly follow
mechanisms B and C.
Fresh
Cycle 1
Cycle 2
100
75
50
25
0
Catalyst recovery
It is known that catalysts following mechanism A are highly
sensitive toward the metals and ligands that are used to syn-
thesize the catalysts.85 Therefore, we focused on the recycla-
bility of catalysts 1c and 2c, that were recovered and re-used
over two more cycles (Figure 7), since we were interested in
examining whether the structure of the catalyst was impor-
tant during the radical pathways (mechanisms B and C) or
not. The amounts of catalyst recovered were low due to
mechanical loss during sampling and the small amounts of
catalysts used. The selectivity profiles toward alcohols,
ketones, aldehyde, and acid are shown in Figures 8 and 9 for
both series of catalysts. For catalyst 1c, the fresh and cycle 2
provided comparable n-octane conversions; however, only
octanones were detected during recycle 1. The 31P NMR
spectra imply slight differences in the catalyst structure with
the appearance of a new peak (for catalyst 2c) (Figure 10). It
can be postulated that the ligand backbone remains
unchanged since the characteristic phosphorous peak for
both the fresh and recovered catalysts is observed. However,
the appearance of a phosphorous peak (2.04parts per million
(ppm)) in the 31P NMR spectrum of the recovered catalyst 2c
may indicate the formation of a new intermediate species
during the catalytic cycle that promotes ketone formation.
The melting points of the recovered catalyst 1c (166–168°C)
and used catalyst mixture (262–263°C) are also different,
which can explain the difference in the selectivity. For cata-
lyst 2c, the fresh run and recovered cycles are comparable,
with only cycle 1 being slightly different in the octanone
selectivity. These tests showed that the mechanism was not
highly dependent on the structures of the catalysts, suggest-
ing the involvement of mechanism B.
Octanols
Octanones
Octanal
Products of oxidation
Figure 9. Recyclability testing of catalyst 2c: selectivity for the
products of oxidation.
Conditions: catalyst/substrate (1:100); substrate/oxidant (1:5); solvent:
DCE; 80°C.
Conclusion
Five different aminodiphosphine ligands were complexed
with Ir and Rh as the metals, and were applied for the oxida-
tion of n-octane using TBHP as the oxidant. The ligands
were selected based on their chain length/steric hindrance,
electron density, and their ability to yield different bite
angles. Both catalysts gave relatively good n-octane conver-
sion. In general, Ir-based complexes catalyze the deep oxida-
tion pathway and they yielded ketones and acids as the major
products, while following the inner-sphere mechanism, and
involvement of M-alkyl radicals during the reaction.
Interestingly, this mechanism seems to also promote the acti-
vation of internal carbons and ketone formation. Rh-based
catalysts showed better selectivity for alcohols than the Ir
complexes, suggesting a slightly greater contribution of the
outer-sphere mechanism (A) (however, mechanism B is still
dominant in this case as well) and forms more alcohols,
while facilitating terminal carbon activation with high com-
mercial values as starting materials for α-olefin formation.
This study showed that mechanismAis sensitive to the struc-
ture of the complexes and the employed metals [high alcohol
selectivity using sterically hindered ligands (hexyl and iso-
propyl)]. In contrast, the catalysts following mechanism B
are not structure-sensitive, and can be recycled with the same
performance, in spite of some structural changes to the cata-
lyst during different cycles of the reaction.
Ketone formation, due to deeper oxidation, is common
when TBHP (t-BuOOH) is used as an oxidant.20,87 It can be
postulated that the reaction mainly proceeds through the
formation of hydroxyl or free radicals (mechanisms B and
C), especially in the case of the Ir catalysts.84,88

Naicker et al.
7
Figure 10. 31P NMR spectra of the fresh and recovered catalysts 1c and 2c.
remaining TBHP and alkylperoxides which are formed as
primary products in alkane oxidation).43 An aliquot (0.5μL)
was injected into the gas chromatograph (GC) and quanti-
fied. A preliminary experiment showed that the standard,
pentanoic acid, is not affected by the reaction medium.
Experimental
Material and methods
All experiments were performed using standard Schlenk
techniques under inert conditions in moisture-free glassware
with anhydrous solvents. All solvents were analytical grade.
To render the reaction glassware moisture-free, it was heated
with a heat gun followed by cycles of vacuum and nitrogen
pressure. Diethyl ether and hexane were distilled from
sodiumbenzophenoneketylundernitrogen.Dichloromethane
was distilled from P2O5, and ethanol from magnesium turn-
ings. Deuterated solvents were used as received and stored in
a desiccator. The NMR spectra were recorded at 162MHz
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect
to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: The
authors gratefully acknowledge the financial support from the
National Research Foundation (NRF), Technology and Human
Resources for Industry Programme (THRIP) (grant no.
1208035643) and the University of KwaZulu-Natal (University
Research Fellowship (URF)).
(31P) using a Bruker Ultrashield 400MHz spectrometer. 31
NMR chemical shifts are reported in ppm from triphe-
nylphosphine (17.6ppm). All PNP ligands were synthesized
using a procedure reported by our group.74
P
Oxidation of n-octane
ORCID iD
All products of the n-octane oxidation were analyzed using a
PerkinElmerAuto System gas chromatograph connected to a
flame ionization detector (FID) set at 260°C. A PONA col-
umn (50m×0.20mm×0.5μm) was utilized with the injec-
tor temperature set at 240°C. Catalytic testing was carried
out in a two-necked pear-shaped flask charged with 10mg of
the respective catalyst, pentanoic acid (as an internal stand-
ard), n-octane, the respective oxidant, and 10mL of the sol-
vent. The flask was equipped with a reflux condenser, and
the contents were stirred, heated to the required temperature,
and maintained at this temperature for 48h in an oil bath.
After that time period, an aliquot was removed using a
Pasteur pipette and filtered through cotton wool and a silica
gel plug, after which PPh3 was added (for reduction of the
Saba Alapour
https://orcid.org/0000-0003-1334-1552
Supplemental material
Supplemental material for this article is available online.
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