Application of new Ru (II) pyridine-based complexes in the partial oxidation of n-octane

Article abstract of DOI:10.1002/aoc.5361

Tridentate and bidentate Ru (II) complexes were prepared through reaction of four pyridine-based ligands: pyCH₂N(R)CH₂py {R = propyl, tert-butyl, cyclohexyl and phenyl; py = pyridine} with the [(η⁶-C₆H₆)Ru(μ-Cl)Cl]₂ dimer. Crystal structures of the new terdentate Ru (II) complexes [Ru{pyCH₂N(R)CH₂py}C₆H₆](PF₆)₂ (R = C₃H₇ (1), C (CH₃)₃ (2), C₆H₁₁ (3) and the bidentate Ru (II) complex [Ru{pyCH₂N(R)}C₆H₆]PF₆ (R = C₆H₅ (4)) are reported. It was found that complexes 1, 2, 3 and 4 crystallised as mono-metallic species, with a piano stool geometry around each Ru centre. All complexes were active in the selective oxidation of n-octane using t-BuOOH and H₂O₂ as oxidants. Complexes 2 and 4 reached a product yield of 12% with t-BuOOH as oxidant, however, superior yields (23–32%) were achieved using H₂O₂ over all systems. The selectivity was predominantly towards alcohols (particularly 2-octanol) over all complexes using t-BuOOH and H₂O₂ after reduction of the formed alkylhydroperoxides in solution by PPh₃. High TONs of up to 2400 were achieved over the Ru/H₂O₂ systems.

Full text of DOI:10.1002/aoc.5361

Received: 25 July 2019
DOI: 10.1002/aoc.5361
Revised: 15 October 2019
Accepted: 17 October 2019
F U L L P A P E R
Application of new Ru (II) pyridine-based complexes in the
partial oxidation of n-octane
Revana Chanerika | Holger B. Friedrich
| Mzamo L. Shozi
School of Chemistry and Physics,
Abstract
University of KwaZulu-Natal, Durban,
South Africa
Tridentate and bidentate Ru (II) complexes were prepared through reaction of
four pyridine-based ligands: pyCH N(R)CH py {R = propyl, tert-butyl,
2
2
Correspondence
6
cyclohexyl and phenyl; py = pyridine} with the [(η -C H )Ru(μ-Cl)Cl] dimer.
Holger B. Friedrich, School of Chemistry
and Physics, University of KwaZulu-Natal.
Durban, South Africa.
6
6
2
Crystal structures of the new terdentate Ru (II) complexes [Ru{pyCH N(R)
2
CH py}C H ](PF ) (R = C H (1), C (CH ) (2), C H (3) and the bidentate
2
6
6
6 2
3
7
3 3
6
11
Email: friedric@ukzn.ac.za
Ru (II) complex [Ru{pyCH N(R)}C H ]PF (R = C H (4)) are reported. It was
2
6
6
6
6
5
Funding information
found that complexes 1, 2, 3 and 4 crystallised as mono-metallic species, with a
cꢀchange, Grant/Award Number: PAR06;
National Research Foundation, Grant/
Award Number: 111508
piano stool geometry around each Ru centre. All complexes were active in the
selective oxidation of n-octane using t-BuOOH and H O as oxidants. Com-
2 2
plexes 2 and 4 reached a product yield of 12% with t-BuOOH as oxidant, how-
ever, superior yields (23–32%) were achieved using H O over all systems. The
2
2
selectivity was predominantly towards alcohols (particularly 2-octanol) over all
complexes using t-BuOOH and H O2 after reduction of the formed
2
alkylhydroperoxides in solution by PPh . High TONs of up to 2400 were
3
achieved over the Ru/H O systems.
2
2
K E Y W O R D S
alcohols, bidentate, NNN ligands, oxidation, ruthenium complexes
1
| INTRODUCTION
be activated to a C-X functionality, with X being either, I,
Br, Cl, OTf, OTs, etc. Alkanes are among the least reac-
tive class of organic hydrocarbons as they constitute of
C-H bond activation or functionalization has been a mat-
ter of interest over several years in an attempt to establish
new grounds from a synthetic perspective and also pro-
vide alternative and green methods for the preparation of
organic compounds. In this regard, the design and
development of efficient catalyst systems to activate C-H
bonds is of great importance. Therefore, it is crucial to
understand the stoichiometric or catalytic principles that
influence such transformations. To study such process
requires a careful design of ligands and catalysts suitable
3
3
3
[4]
strong C sp -H bonds and single C sp -C sp bonds.
Transition metals provide
functionalising inert C-H bonds.
a
possible route to
[
5]
[1]
Over the past four decades, much attention has been
drawn to developing homogeneous catalysts for their
advantages over heterogeneous systems, such as milder
reaction conditions, higher selectivity, and an enhanced
[
4]
range of transformation modes. Many desirable proper-
ties arise from the coordination of a multidentate ligand
to a specific metal, such as the reduction of leaching (dis-
sociation of the ligand from the metal). The lability of the
complex may be fine-tuned by the donor atoms and sub-
stituents present on the complex, thereby influencing the
[1]
for the activation of C-H bonds.
Carbon-hydrogen bonds are deemed inert since they
are far less reactive than carbon–oxygen or carbon-halo-
gen moieties.
[2,3]
In this sense, the inert C-H bond should
Appl Organometal Chem. 2019;e5361.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
1 of 13
https://doi.org/10.1002/aoc.5361

2
of 13
CHANERIKA ET AL.
metal electron density as well as the coordination of sub-
strates to the metal. This controls the electronic and
steric properties of the metal as a whole.
complexes with N-donor ligands, where we have previ-
ously reported analogous Co (II) tridentate complexes.
[6]
[23]
The present work investigated the catalytic activity of
four new pyridine-based ruthenium systems in the partial
oxidation of n-octane. It further investigated the stability,
steric effects, and electronic effects of the complexes by
varying the N-donor backbone from propyl, tert-butyl,
cyclohexyl and phenyl groups as depicted in Figure 1.
Ligands containing nitrogen donors have gained
attention in various fields, including homogeneous catal-
ysis, coordination chemistry and organic synthesis.
Research conducted in recent years has revealed applica-
tions of planar tridentate NNN ligands in materials, phys-
ical chemistry, homogenous catalysis and organic
[7–9]
synthesis.
In contrast, reports on unsymmetrical pla-
nar tridentate NNN ligands bound to various transition
metals have only been sporadic.
2 | EXPERIMENTAL
2.1 | General
[10]
Recent developments in ruthenium pincer complexes
have shown new reactivities in activating strong chemical
bonds and several serve as efficient catalysts for various
All experimental manipulations of the ruthenium com-
pounds were carried out under inert gas using standard
Schlenk techniques unless otherwise noted. All reaction
glassware was oven dried. Solvents were dried prior to
use according to established methods.
lized in ligand synthesis, including 2-pyridine methanol
[11]
reactions, including green transformations.
Zeng and
Yu have reported efficient hemilabile Ru (II) NNN com-
plexes
incorporating
an
unsymmetrical
2-
[
13]
(
benzoimidazol-2-yl)-6-(pyrazole-1-yl) pyridine ligand in
Chemicals uti-
[12]
the transfer hydrogenation of ketones.
A report by
Sarkar et al. describes bidentate and tridentate Ru (II)
carbonyl complexes bearing thioarylazoimidazole
ligands, with the bidentate catalyst exhibiting the highest
activity in the oxidation of alcohols by N-
(98%), propylamine (98%), tert-butylamine (98%), cyclo-
hexylamine
(99%),
aniline
(99%)
and
tetrabutylammonium bromide (TBAB), were purchased
from Sigma-Aldrich and used as received. Ruthenium
(III) chloride hydrate was sourced from DLD Scientific.
The two oxidants, t-BuOOH and H O , were purchased
[13]
methylmorpholine-N-oxide.
Recently we reported new
5
(
η -cyclopentadienyl)dicarbonylruthenium (II) amine
2
2
complexes in the oxidation of styrene, with a dinuclear
from Sigma Aldrich and DLD Scientific, respectively. The
H and C NMR spectra of all ligands and diamagnetic
ruthenium compounds were recorded using a Bruker
Avance 400 MHz spectrophotometer and are reported as
chemical shifts (δ, ppm) with reference to the solvent
5
1
13
complex, [(η -C H )Ru (CO) NH (CH ) ] [BF ] , demon-
5
5
2
2
2 3 2
4 2
[
14]
strating the best activity with yields reaching 95%.
In a
0
further study, Ru (II) N,N -bidentate complexes incorpo-
rating pyridyl-imine ligands for the transfer hydrogena-
tion of cyclohexanone were reported with high yields and
peak, dimethylsulfoxide-d6 (m, 2.50 ppm). Chemical
[
15]
13
TONs of 1990.
shifts (δ, ppm) of peaks in the proton decoupled
C
To the best of our knowledge, there are few to no
reports on NNN-pyridine based Ru (II) complexes in the
activation of paraffinic C-H bonds, however, the use of a
few related compounds in several other applications has
NMR data are referenced to the DMSO-d solvent peak
6
(39.51 ppm) with the specific carbon indicated in paren-
theses. IR spectra were recorded using a Perkin Elmer
Attenuated Total Reflectance (ATR) spectrophotometer
[16–22]
−1
been reported.
As part of our continuing work on
between 4000–380 cm . Elemental analyses were
FIGURE 1 Synthesis of Ru[pyCH
CH py]C (PF (1–3) and Ru[pyCH
ClC PF (4)
2
N(R)
2
NH(R)]
2
6
H
6
6
)
2
6
H
6
6

NEW RU(II) PYRIDINE-BASED COMPLEXES IN THE PARTIAL OXIDATION OF OCTANE
3 of 13
−
1
recorded on a ThermoScientific Flash 2000 CHNS/O ana-
lyzer. High and low resolution mass spectrometric data
were obtained on a Bruker Micro TOF-Q11 using an elec-
tron spray ionisation (ESI) technique, with a sample con-
py), 160.5 (C-py). IR νmax (cm ): 1609 (s) (C-C aromatic),
1440 (s) (C-H alkyl), 779 (s) (C-H rocking). m/z (calcd):
210 (210.56). Melting point: 220.7–224.8 C. Elemental
ꢁ
analysis for C H N RuP F : calcd C, 35.5; H, 3.6; N,
21
25
3
2 12
centration
of
10
ppm.
The
ligands
L1
5.9; found C, 35.6; H, 3.6; N, 6.1.
[
[
pyCH NC H CH py], L2 [pyCH NC(CH ) CH py], L3
2
3
7
2
2
3 3
2
pyCH NC H CH py], L4 [pyCH NC H CH py] and
2
6
11
2
2
6
5
2
precursor, 2-chloromethylpyridine hydrochloride, were
prepared as described previously.
2.4 | Synthesis of
[23]
[Ru{pyCH NC(CH ) CH py}C H ](PF ) (2)
2
3 3
2
6
6
6 2
Complex 2 was prepared analogously to 1 using L2
6
6
2
.2 | Synthesis of the [(η -C H )Ru(μ-Cl)
(0.20 g, 0.78 mmol); [(η -C H )Ru(μ-Cl)Cl] (0.20 g,
6
6
6
6
2
Cl] dimer
0.39 mmol) and NH PF (0.13 g, 0.78 mmol) was added
2
4 6
to the yellow colored solution. The obtained mustard
powder was dried in vacuo for several hours and rec-
rystallized upon vapor diffusion of diethyl ether into a
concentrated acetonitrile solution, yielding crystals suit-
6
The [(η -C H )Ru(μ-Cl)Cl] dimer was synthesised in a
6
6
2
[
24]
similar manner to the report by Bennet and Smith.
a 100 ml nitrogen saturated round bottom flask, 0.5 g
RuCl .xH O was added in 25 ml ethanol. The mixture
To
1
able for X-ray diffraction (0.27 g, 94%). H NMR (DMSO):
3
2
was allowed to stir, after which 2.5 ml of 1,4-
cyclohexadiene was added. The brown precipitate, which
formed after a 4 hr reflux, was collected under vacuum
filtration and washed with a small portion of methanol.
The resultant solid was dried in vacuo.
δ 1.6 (s, 9H, CH -tbut), 3.9; 4.9 (d, 2H, J = 17.3 Hz; d, 2H,
3
J = 17.2 Hz, N-CH -py), 6.4 (s, 6H, C H ), 7.6 (ddd, 2H,
2
6
6
J = 7.7, 6.5, 0.9 Hz, CH-py), 7.5 (d, 2H, J = 7.8 Hz, CH-
py), 8.0 (ddd, 2H, J = 8.8, 7.7, 1.0 Hz, CH-py), 9.3 (d, 2H,
13
J = 6.2 Hz, CH-py). C NMR (DMSO): δ 63.4 (CH -tbut),
3
6
8.7 (N-CH -py), 88.5 (C H ), 121.4 (CH-py), 122.6 (CH-
2 6 6
py), 135.8 (CH-py), 148.1 (CH-py), 161.9 (C-py). IR ν
(cm ): 1613 (w) (C-C aromatic), 1448 (w) (C-H alkyl),
max
−1
2
.3 | Synthesis of
[Ru{pyCH NC H CH py}C H ](PF ) (1)
779 (s) (C-H rocking). m/z (calcd): 217.56 (217.06). Melt-
ing point: 287.1–288.7 C. Elemental analysis for
2
3
7
2
6
6
6 2
ꢁ
To a 100 ml round bottom flask containing a mixture of
C H N RuP F : calcd C, 36.5; H, 3.8; N, 5.8; found C,
36.0; H, 3.6; N, 5.2.
22
27
3
2 12
6
L1 (0.29 g, 1.20 mmol) in 40 ml of methanol, [(η -C H )
6
6
Ru(μ-Cl)Cl] (0.3 g, 0.6 mmol) was added. The mixture
2
was allowed to stir at room temperature for 24 hr during
which the color of the solution changed to a dark-brown
color. The solution was concentrated to ~7 mL after
which NH PF (0.20 g, 1.20 mmol) was added and stirred
2.5 | Synthesis of
[Ru{pyCH NC H CH py}C H ](PF ) (3)
2
6
11
2
6
6
6 2
4
6
for 1 hr. The resulting precipitate was filtered off and
washed with a small portion of cold methanol and
diethyl ether. The crude product was purified by dis-
solving the solid in acetonitrile, filtering off the
undissolved material and precipitating the product with
diethyl ether. The resulting dark-green crystalline solid
was dried in vacuo for several hours and recrystallized by
vapor diffusion of diethyl ether into a concentrated aceto-
nitrile solution, yielding crystals suitable for X-ray dif-
Complex 3 was prepared analogously to 1 using L3
6
(0.20 g, 0.71 mmol); [(η -C H )Ru(μ-Cl)Cl] (0.18 g,
6
6
2
0.36 mmol) and NH PF (0.17 g, 0.71 mmol) was added
4
6
to the yellow colored solution. The obtained yellow pow-
der was dried in vacuo for several hours and rec-
rystallized upon vapor diffusion of diethyl ether into a
concentrated acetonitrile solution, yielding crystals suit-
1
able for X-ray diffraction (0.21 g, 79%). H NMR (DMSO):
δ 1.2 (m, 2H, CH -cy), 1.6 (q, 2H, J = 9.1 Hz, CH -cy), 1.7
2
2
1
fraction (0.32 g, 75%). H NMR (DMSO): δ 1.0 (t, 3H,
(m, 2H, CH -cy), 1.8 (m, 2H, CH -cy), 2.2 (m, 2H, CH -
2 2 2
J = 7.1 Hz, CH -prop), 1.8 (m, 2H, CH -prop), 4.0 (m,
cy), 4.1 (m, 2H, CH -cy), 4.0; 4.8 (d, 2H, J = 17.3 Hz; d,
3
2
2
2
H, prop-CH -N), 4.4; 4.8 (d, 2H, J = 16.9 Hz; d, 2H,
2H, J = 15.6 Hz, N-CH -py), 6.3 (s, 6H, C H ), 7.6 (ddd,
2
2
6
6
J = 17.4 Hz, N-CH -py), 6.3 (s, 6H, C H ), 7.5 (m, 2H,
2H, J = 7.8, 6.8, 1.0 Hz, CH-py), 7.6 (d, 2H, J = 7.8 Hz,
CH-py), 8.1 (ddd, 2H, J = 8.6, 8.0, 1.5 Hz, CH-py), 9.3 (d,
2H, J = 5.6 Hz, CH-py). C NMR (DMSO): δ 24.2 (CH2-
2
6
6
CH-py), 7.4 (d, 2H, J = 7.5 Hz, CH-py), 7.9 (ddd, 2H,
J = 8.8, 7.7, 1.0 Hz, CH-py), 9.2 (d, 2H, J = 5.6 Hz, CH-
13
13
py). C NMR (DMSO): δ 10.9 (CH -prop), 17.8 (CH -
cy), 25.2 (CH -cy), 25.4 (CH -cy), 28.0 (CH -cy), 32.9
3
2
2
2
2
prop), 68.4 (CH -N-py), 70.1 (prop-CH -N), 88.3 (C H ),
(CH -cy), 74.0 (N-CH -py), 59.3 (CH-cy), 89.5 (C H ),
2 2 6 6
2
2
6
6
1
22.4 (CH-py), 125.6 (CH-py), 140.3 (CH-py), 154.4 (CH-
123.0 (CH-py), 126.3 (CH-py), 141.0 (CH-py), 155.6 (CH-

4
of 13
CHANERIKA ET AL.
−
1
py), 162.3 (C-py). IR νmax (cm ): 1611 (w) (C-C aro-
hydrogen atoms, followed by anisotropic refinement by
2
matic), 1461 (w) (C-H alkyl), 771 (s) (C-H rocking). m/z
calcd): 230.57 (230.06). Melting point: 237–241.3 C. Ele-
mental analysis for C H N RuP F : calcd C, 38.4; H,
.9; N, 5.6; found C, 38.8; H, 4.0; N, 5.5.
full-matrix least-squares methods based on F using
ꢁ
[28]
(
SHELXL.
Structural diagrams and publication mate-
[
28]
[29]
rial were generated using SHELXL,
PLATON,
X-
24
29
3
2 12
[
30,31]
[32]
[33]
3
Seed,
POV-Ray
and MERCURY.
Reports were
subjected to online check cif validation showing no struc-
tural disorder in 1–4. Supplementary crystallographic
data for compounds 1–4 can be obtained from CCDC
2
.6 | Synthesis of [Ru{pyCH NHC H }
2 6 5
C H ]PF (4)
1940854–1940857
via
www.ccdc.cam.ac.uk/data_
6
6
6
request/cif.
Complex 4 was prepared analogously to 1 using L4
6
(
0.20 g, 0.73 mmol); [(η -C H )Ru(μ-Cl)Cl] (0.18 g,
6
6
2
0
.36 mmol) and NH PF (0.12 g, 0.73 mmol) was added
2.8 | Oxidation of n-octane
4
6
to the brown colored solution. The dark-brown crystal-
line solid was dried in vacuo for several hours and rec-
rystallized upon vapor diffusion of diethyl ether into a
concentrated acetonitrile/methanol solution, yielding
crystals suitable for X-ray diffraction (0.26 g, 95%). H
NMR (DMSO): δ 4.4; 5.2 (d, 1H, J = 14.9 Hz; m, 1H,
Paraffin oxidation studies were performed under inert
conditions in moisture free glassware using dry solvents.
The n-octane to compound ratio was kept constant at
1:100 unless otherwise stated. A mass of 10 mg of cyclo-
pentanone as the internal standard was loaded each time
into a 50 ml two-neck pear shaped flask. Each mixture
comprised of ~6 mg of Ru complexes, 1–3, or ~3 mg of
the bidentate complex 4, in 10 ml of acetonitrile, together
with the hydrocarbon substrate (0.008 mmol) and inter-
nal standard (~10 mg). The oxidants (t-BuOOH or H O )
1
N-CH -py), 5.4 (s, 6H, C H ), 7.4 (m, 1H, CH-ph), 7.5
2
6
6
(d, 1H, J = 7.8 Hz, CH-py), 7.6 (m, 1H, CH-py), 7.6 (m,
4
9
5
H, CH-ph), 8.1 (ddd, 1H, J = 8.7, 7.6, 1.5 Hz, CH-py),
.2 (d, 1H, J = 5.9 Hz, CH-py). C NMR (DMSO): δ
13
8.9 (N-CH -py), 87.2 (C H ), 119.3 (CH-ph), 125.2
2
6
6
2 2
(
1
CH-py), 126.0 (CH-py), 129.4 (CH-ph), 139.6 (CH-py),
55.1 (CH-py), 159.5 (C-py). IR νmax (cm ): 1597 (w)
were added at differing ratios relative to the amount of
substrate added (1:3, 1:6, 1:9, 1:12, 1:15 for t-BuOOH and
1:6, 1:9, 1:12 for H O ) from which the optimum ratio of
−
1
(C-C aromatic), 1427 (w) (C-H alkyl), 763 (s) (C-H
2
2
rocking). m/z (calcd): 399.02 (399.02). Melting point:
1:12 for the Ru/t-BuOOH (~32 mmol) and Ru/H O
2 2
ꢁ
2
37.6–241.4 C. Elemental analysis for C H N
(~42 mmol) complexes, respectively, were established. At
pre-established optimum conditions, an aliquot of the
mixture was removed with a Pasteur pipette and PPh3
was added prior to filtering through silica. Of this mix-
ture, 0.5 μL was injected into a Perkin Elmer Claurus 580
Auto System Gas chromatograph integrated with a Flame
Ionisation Detector (FID) for quantification of the prod-
uct stream. All data are a result of an average of two indi-
vidual runs with differences reported to within 1%.
Column specifications and GC parameters are listed in
the supplementary information (Table S1). Studies inves-
tigating the mechanism of the catalytic system were done
18
18 2
RuClPF : calcd C, 39.7; H, 3.3; N, 5.2; found C, 40.0; H,
6
3
.3; N, 5.6.
2.7 | X-ray analyses
Selected crystals of complexes 1–4 were glued onto the
tips of glass fibers and mounted in a stream of cold nitro-
gen (173 K). Each crystal was centered in the X-ray beam
with the aid of a video camera. Single crystal X-ray dif-
fraction data and evaluation was performed on a Bruker
Smart APEXII diffractometer with graphite mono-
chromated Mo Kα radiation (50 kV, 30 mA and
using
the
radical
scavenger,
2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) with five equiva-
lent additions relative to the substrate.
[25]
λ = 0.71073 Å) using the APEXII
data collection soft-
ware. Data collection was carried out at 100(2) K and the
temperature was controlled by an Oxford Cryostream
cooling system (Oxford Cryostat) operating at 100(1) K.
3 | RESULTS AND DISCUSSION
ꢁ
The collection method involved ω-scans of width 0.5
and 512 x 512 bit data frames. Data reduction and cell
The synthesis and characterization of four new Ru com-
plexes are described in which 1–3 contain a di-pyridyl
NNN ligand system, with R groups, propyl, tert-butyl and
cyclohexyl, on the central N-donor atom that is bound to
two constrained six membered pyridine rings via two
individual methylene linkers. Complex 4 incorporates a
[
26]
refinement were performed using the program SAINT.
The data were scaled and absorption corrections per-
[27]
formed using the SADABS
multi-scan technique. The
structures were solved by direct methods using
[28]
SHELXS.
Isotropic refinement was first done on non-

NEW RU(II) PYRIDINE-BASED COMPLEXES IN THE PARTIAL OXIDATION OF OCTANE
5 of 13
single pyridine ring linked to a phenyl substituted N-
donor backbone via one methylene group.
The formation of the intended compounds was indi-
cated through color changes observed during each reac-
6
tion with the [(η -C H )Ru(μ-Cl)Cl] precursor and
6
6
2
respective ligand. The structures and geometries of these
compounds were established through single crystal X-ray
diffraction, together with elemental analysis, IR and
NMR spectroscopies. Noted differences between the melt-
ing points of 1–4 versus their starting materials also
showed successful complexation.
Ru (II) forms a diamagnetic low spin complex and as
a result NMR studies could be easily carried out. The aro-
matic proton signals of 1–4 show significant shifts rela-
1
tive to those observed in the H NMR spectra of the
[
34]
uncoordinated ligands, confirming coordination.
FIGURE 2 Single crystal structure of complex 1 drawn at 50%
thermal ellipsoid probability, with omission of hydrogen atoms for
clarity
Apart from these resonance shifts, distinct singlets
around 5.5–6.4 ppm, due to the protons of the benzene
ring in the Ru complexes, further confirms successful
complex synthesis. The singlet, centered around 3.7–
4
.3 ppm in the free ligands (assigned to the methylene
complexes, which shows coordination of the pyridine
−
protons), splits into two doublets on coordination, with
the first at around 3.9–4.4 ppm and the other at 4.8–
rings to the Ru metal. Furthermore, an intense PF band
6
−
1 [44]
is seen at 828 cm .
All other bands seen in the IR
5
.2 ppm, due to these becoming diastereotopic
spectra of the Ru complexes were low in intensity. Also,
synthesis of these complexes was further confirmed by
mass spectrometry and single crystal XRD results, shown
later.
[35–38]
(Table 1).
Moreover, the methylene linker separat-
ing the aliphatic/aromatic and pyridine fragments of the
ligand is affected by the planarity of the substituent on
the central N-donor atom. Therefore, the split CH sig-
2
nals are also due to the pyridine rings and central nitro-
gen substituents appearing in two different planes to
each other.
3.1 | Crystal structures of 1–4
[39]
Compound 4 forms a bidentate complex which was
confirmed by X-ray diffraction, as well as NMR, MS and
elemental analysis. A few examples of similar chemistry
The Ru complexes with the different N-bonded R groups
[
45]
(1–4) all exhibit a piano-stool geometry
(Figures 2–5),
with 1–3 crystallizing in the monoclinic P2 /c space
1
[40,41]
are known,
dentate ligand is rare but not unknown.
and cleavage of a side arm of a multi-
group and 4 in P2 /n. All complexes exist as monomeric
1
[42]
units due to the metal ion being stabilized through elec-
tron donation from the nitrogen atoms and aliphatic moi-
eties on the central N-donor backbone, thus preventing
dimerization.
The Ru atom in 1–3 binds to the three N-donor atoms
in a terdentate fashion, and to the electron rich benzene
ring. The NNN coordinating ligand occupies the positions
as the legs of the piano stool, whilst the benzene ring,
Apart from a clear shift in the C-H alkyl stretch and
bend, C-C aromatic and C-H rocking vibrational bands
between the coordinated and uncoordinated ligands (sup-
plementary information, Table S2), it is worth noting that
the C=N vibration is generally strong, as opposed to the
corresponding peak appearing in the uncoordinated
[43]
ligand (Table 1).
This observation is noticeable in all
1
TABLE 1
H NMR and IR shifts of the complexes relative to those observed in the ligands
1
-1
H NMR IR νmax/cm
Complex
Ligand (CH
2
)
Complex (CH
2
)
Ligand(C=N)
1667
Complex (C=N)
1
2
3
4
3.7
3.8
3.8
4.3
4.4, 4.8
1609
1613
1611
1602
3.9, 4.9
1589
4.0, 4.8
1588
4.4, 5.2
1597

6
of 13
CHANERIKA ET AL.
FIGURE 3 Single crystal structure of complex 2 drawn at 50%
thermal ellipsoid probability, with omission of hydrogen atoms for
clarity
FIGURE 5 Single crystal structure of complex 4 drawn at 50%
thermal ellipsoid probability, with omission of hydrogen atoms for
clarity
occupying the remaining coordination sites, takes up the
position as the seat of the piano stool. The Ru center in 4
is coordinated to the nitrogen atoms of the pyridine and
the phenyl ring, and a chlorine atom as the base of the
stool, with the benzene occupying the apex of the stool.
Selected bond lengths and angles of crystal structures
distance is 1.470 Å, showing that the benzene ring in this
case is bound more weakly to the metal ion with a more
limited flow of electrons from the benzene ring to the
[
20]
metal.
In 3, the N(1)-Ru(1) distance of 2.177(4) Å is shorter
in comparison to the distance (2.242(3) Å) in complex 2,
suggesting that the Ru may be more electron rich. The
1–3 are given in Table 2. Crystallographic and structure
refinement data for all complexes are presented in the
supporting information (Table S3). The Ru-Cg distance
N(2)-Ru(1)-N(1) bite angle is more acute for 3
(76.22(18) ) than for 2 (79.23(18) ) and 1 (80.63(10) ),
ꢁ
ꢁ
ꢁ
(
the distance of the Ru to the midpoint of the benzene
which tend more closely to the tridentate facial coordina-
tion angle of 90 . This implies greater steric hindrance
around the metal center in 3, with a less open
ꢁ
ring) of 1.462 Å for 1 is comparable to the Ru-Cg distance
of 1.463 Å for 3. However, in complex 2, the Ru-Cg
FIGURE 4 Single crystal structure of
complex 3 drawn at 50% thermal ellipsoid
probability, with omission of hydrogen atoms for
clarity

NEW RU(II) PYRIDINE-BASED COMPLEXES IN THE PARTIAL OXIDATION OF OCTANE
7 of 13
ꢁ
TABLE 2 Selected bond lengths (Å) and angles ( ) for
The crystal structure of 1 shows non-covalent interac-
complexes 1–3
tions of C-HꢂꢂꢂF bonds (supplementary information, Fig-
ure S1). The PF6− counter ion is involved in C-HꢂꢂꢂF non-
covalent intermolecular interactions. Non-classical inter-
molecular interactions are observed between two F atoms
on the PF6− counterion and two H atoms on the pyridine
ring. The HꢂꢂꢂF distances in these interactions are 2.382
1
2
3
N(1)-Ru(1)
2.173(2)
2.098(3)
2.082(3)
79.05(10)
80.63(10)
79.24(10)
2.242(3)
2.082(3)
2.089(3)
78.66(12)
79.23(12)
78.66(12)
2.090(5)
2.177(4)
2.091(5)
88.66(19)
76.22(18)
88.66(19)
N(2)-Ru(1)
N(3)-Ru(1)
ꢁ
N(2)-Ru(1)-N(3)
N(2)-Ru(1)-N(1)
N(3)-Ru(1)-N(1)
and 2.519 , respectively with C-HꢂꢂꢂF angles of 151.69
ꢁ
and 131.43 , respectively. The second interaction exists
−
between two benzene hydrogen bonds and the PF6 (F)
atom with HꢂꢂꢂF distances of 2.584 and 2.487 , respec-
ꢁ
ꢁ
tively and C-HꢂꢂꢂF angles of 120.38 and 124.43 , respec-
coordination sphere to potential binding substituents
compared to the geometry of complexes 1 and 2. The
trend observed provides a direct correlation between the
steric size of the central N-donor substituents and the bite
angles, which relates to the ability of the metal ion to
accommodate and coordinate substrates during catalytic
tively (Figure S1).
3.2 | Oxidation of n-octane catalyzed by
1–4
[46–48]
transformations.
Furthermore, the dissection angle
All Ru complexes were tested in the C-H bond activation
defined by the planes of the [N(1), C(7), C(8), C(9), C(10),
C(11)] and [N(3), C(20), C(21), C(22), C(23), C(24)] pyr-
idyl rings is seen to be acute at 88.31 . This deviates sig-
of n-octane using two oxidants, t-BuOOH (70% in H O)
2
and H O (30% in H O). Reactions were tracked by GC
2
2
2
ꢁ
and PPh3 was added (to reduce any hydroperoxides)
when reactions were stopped at either 12 or 27 hr when
reactions were deemed complete. Results reported are
nificantly to the equivalent angles of complexes 1 and 2,
ꢁ
ꢁ
with angles of 63.13 and 63.34 , respectively. This effect
is rationalized through twisting of the methylene linker
on the side of the [N(3), C(20), C(21), C(22), C(23) and
C(24)] pyridyl ring.
after PPh addition and from a series of experiments
3
investigating product yields, the optimum octane to oxi-
dant ratio using t-BuOOH (~32 mmol) and H O
2
2
In complex 4, the arrangement of the chlorine atom is
oriented on the same side as the phenyl moiety due to
the reduced steric bulk of this complex. The inter-
secting angle between the planes of the phenyl group
(~42 mmol) was established at 1:12. The optimum tem-
perature for all reactions employing both oxidants was
80 C, with the highest oxygenate selectivity occurring by
15
ꢁ
12 and 27 hr for the Ru/t-BuOOH and Ru/H O systems,
2
2
[
C(7), C(8), C(9), C(10), C(11) and C(12)] and the Ru(1)-
respectively. Under the established optimum conditions,
blank reactions with no added complex were carried out,
where a yield of 5% was seen using t-BuOOH, whilst a 4%
yield was noted for reactions employing H O . Further-
ꢁ
N(1)-C(13)-C(14)-N(2) metallacycle of 45.91 is similar to
15
those reported for related compounds. In this complex,
the benzene ring is bound most weakly to the Ru centre
with a Ru-Cg distance of 1.664 Å. Apart from a reduced
flow of electrons from the benzene moiety to the Ru,
evidenced by their increased distance from each other, a
further reduction in electron density at the Ru metal is
observed due to lengthening of the N(1)-Ru(1) bond
2
2
more, blank reactions with the Ru precursor gave yields
of 13% and 6% for the t-BuOOH and H O2 systems,
2
respectively (Table 4). In the Ru/t-BuOOH reactions,
product yields were slightly lower compared to the results
when the Ru salt was used as catalyst and may be attrib-
uted to the ligand induced steric effects around the metal.
Confirmation of the stability of cyclopentanone with t-
BuOOH and H O is shown in the supplementary mate-
(2.159 (2) Å) as shown in Table 3.
ꢁ
2
2
TABLE 3 Selected bond lengths (Å) and angles ( ) for
complex 4
rial (Figure S2).
Results of the catalytic study employing compounds
–3 with t-BuOOH show that compound 2 (entry 4) gives
a yield of 12% to C8 oxygenates after 12 hr, whilst 3
(entry 7) was the least active, with a yield of 7% (Table 4).
All compounds (1–3) were more selective to alcohols over
2 hr (Figure 6A). The benzene ring can dissociate from
the metal (as for example shown by the NMR spectrum
of the recovered catalyst 1 from the reaction of complex
, designated 1R, Figure 7). Thus, compounds 1–4 are
4
1
N(1)-Ru(1)
2.159(2)
N(2)-Ru(1)
2.082(2)
2.3839(7)
82.11(7)
86.07(6)
76.06(9)
Cl(1)-Ru1
1
N1-Ru(1)-Cl(1)
N2-Ru(1)-Cl(1)
N2-Ru(1)-N(1)
1

8
of 13
CHANERIKA ET AL.
TABLE 4 Optimum conditions and catalytic testing of n-octane over 1–4
a
b
c
Entry
Complex
Oxidant
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
Substrate: Oxidant
Complex loading (mol%)
Product yield (mol%)
TON
1
2
3
4
5
6
7
8
9
-
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
1:12
-
5
13
8
-
Ru salt
1
13
8
1
1
2
1
12
9
12
900
90
7
2
0.01
0.1
1
2
9
3
7
4
1
12
4
12
-
-
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
-
1
1
1
1
1
1
1
0
1
2
3
4
5
6
Ru salt
1
6
6
1
2
3
3
3
4
1
23
27
32
24
33
7
23
27
32
2400
330
7
1
1
0.01
0.1
1
a
1
mol compound: 100 mol octane (1 mol%), 1 mol compound: 1000 mol octane (0.1 mol%), 1 mol compound: 10 000 mol octane (0.01 mol%) using
cyclopentanone (~10 mg) as the internal standard and reactions times of 12 and 27 hours for t-BuOOH (32 mmol) and H
2
O
2
(42 mmol), respectively.
b
Total moles of product/initial moles octane.
c
TON = moles total products (mol)/moles of compound (mol).
FIGURE 6 A Product distribution
profile for compounds 1–4 with t-
ꢁ
BuOOH over 12 h at 80 C B Product
distribution profile for compounds 1–4
ꢁ
with H
2
O
2
over 27 hr at 80 C
FIGURE 7 NMR spectrum of the
compound recovered after the reaction
of catalyst 1, (1R)

NEW RU(II) PYRIDINE-BASED COMPLEXES IN THE PARTIAL OXIDATION OF OCTANE
9 of 13
really 18 electron pre-catalysts, that lose benzene to give
the active catalytic species. This allows for easier binding
of the substrate to the metal center, which supports the
observed degree of over-oxidation (Figure 6A). In the case
of 1, the free rotation of the propyl group may allow for
improved accommodation of the substrate and conse-
quently higher yields, which in turn offers less control
over the primary product selectivity, and hence a higher
production of ketones. The production of ketones was
also dominant in the blank reaction (no added complex)
as well as the experiment with the Ru salt as catalyst fur-
ther highlighting the ligand effect. The product distribu-
tion profile for 2 is presented in the supporting
information (Figure S3).
than with t-BuOOH. The product distribution profiles
show high formation of alcohols, once
alkylhydroperoxides formed in solution were reduced by
PPh , over 27 hr (including reactions with no added com-
3
pound and Ru salt) with very low selectivity to ketones
(Figure 6B), which may be due to a slower reaction rate
than with t-BuOOH. The GC profile of the product mix-
ture after workup is shown in Figure S5. Previous reports
have shown a promoting effect of water in the catalysis
[
50–52]
with H O .
Furthermore, the water in H O solu-
2 2
2
2
tions has been suggested to be directly involved in gener-
[
53,54]
ating hydroxyl radicals.
This, however, does not
apply to the t-BuOOH systems, where an experiment
done to investigate this, using 3, added water and t-
BuOOH as oxidant, shows a lower yield of 6% (8% yield
prior to dilution with water) after 15 hours. Overall, the
high yield of oxygenates over complexes 1–3 may be asso-
ciated to the pincer complexes being largely responsible
for stabilizing the Ru-OH intermediate in Scheme 2, prior
to the production of alcohols, and/or promote the forma-
tion of OH radicals.
The activation of the terminal hydrocarbon, C(1),
ꢁ
tends to be very difficult as 2 carbons are substantially
ꢁ
more reactive than 1 carbons. However, compounds 1–3
produce 1-octanol and octanoic acid over 12 hr with t-
BuOOH as oxidant (Table 4). It has been well docu-
mented that catalytic intermediates can be selective when
controlled by free energies of activation and therefore C-
H bond cleavage at the terminal carbon can be achieved
if the system is sufficiently reactive to cleave a strong
Using 2, 2-octanol as a substrate and t-BuOOH as the
oxidant, gave a 100% yield to the corresponding ketone
within 3 hours, whilst oxidation by H O over 3, showed
[15,43,49]
bond.
With H O as oxidant, 3 was most active with a total
2
2
a yield of only 2% after 3 hr. It is thus noted that with the
t-BuOOH systems, over-oxidation is fairly rapid over the
complexes (all of the 2-octanol is oxidized to the ketone
rapidly), however, in the H O systems, over-oxidation is
2
2
product yield of 32% (entry 13, Table 4) at the end of
7 hr. Monitoring the reactions over time showed that
2
the C8 oxygenate production rate decreased until 27 hr
when the formation effectively stopped. n-Octane conver-
sion does continue, however, high boiling side products
form, such as dioctyl ether, from secondary reactions.
The product distribution profile for 3 over 27 h is given in
the supplementary information (Figure S4).
2
2
slow in the presence of the complex.
The reaction of 1-octanol as substrate with H O gave
2
2
1% yield to octanal in the presence of 3 over 9 hours.
Thus, it is seen that deeper oxidation at the terminal car-
bon is very slow (in the presence and absence of complex
with H O ), since the blank reactions (no complex) of the
It is seen that complexes 1–3, with H O , behave dif-
2
2
2 2
ferently than with t-BuOOH. The efficiency of each sys-
tem was influenced by ligand rigidity, as well as
electronic factors, and all systems with H O are active
alcohol also showed yields of only 1% in 9 hr. It thus
seems that the Ru compound with H O causes very little
or no deeper oxidation.
2
2
2
2
over longer reaction times compared to those with t-
The TON data for the oxidation of n-octane in Table 4
with t-BuOOH, shows 2 with a TON of 12, whilst 1 and 3
BuOOH. The complexes with H O showed better results
2
2
SCHEME 2 Proposed mechanism for the
oxidation of n-octane catalysed by 1–4
with H O
2 2

10 of 13
CHANERIKA ET AL.
were the least active with TONs of 8 and 7, respectively.
Looking at the TONs with H O , it is seen that com-
internal carbon positions of the n-octane chain was
observed with reported ratios of 1:1:1.3 and 1.7:1:1.1.
[57]
2
2
pounds 1–3 are between two and four times more effi-
cient in the presence of this oxidant when compared to t-
BuOOH. The most efficient system was 3 with a TON of
Furthermore, the reported selectivity parameters in the
n-octane activation by t-BuOOH was different to litera-
ture values involving OH• but was comparable to those
involving the generation of tBuO•. In another report by
Shul'pin and co-workers investigating the oxidation of n-
octane over NaVO3, high regioselective ratios of
1:10.1:10.7:8.4 and 1:7:6:5 were obtained in MeCN and
H O, respectively employing H O and H SO as co-cata-
3
2. Again, these results show that H O is a better oxidant
2 2
than t-BuOOH, in that the complexes are more active and
efficient over longer reaction periods. Reducing the com-
plex loading to 0.1 mol% and 0.01 mol% of 2 and 3,
respectively, show that a complex loading of 0.01 mol%
with t-BuOOH gives a TON of 900. With H O , a com-
2
2
2
2
4
[
58]
lysts.
With the observed selectivity parameters, the
2
2
pound loading of 0.01 mol% gave a TON of 2400.
octane oxidation reaction was thought to proceed via the
formation of OH•. Another study investigating the oxida-
tion by t-BuOOH of n-octane over SNS ligand catalysts
with varying N-donor backbones (pyridine and amine)
revealed a high selectivity to octanones over the amine-
based SNS catalyst with regioselective ratios of 1.4:1.1:1
to the C(2), C(3) and C(4) positions. In contrast, pyridine-
based complexes were more selective to the octanol, spe-
Table 5 illustrates the regioselective parameter C(1):
C(2):C(3):C(4) over all Ru complexes, which provides
information on the reactivity of the hydrogens at carbon
positions 1, 2, 3 and 4 of the hydrocarbon chain. These
are normalized by accounting for the number of hydro-
[55]
gens present on each carbon atom.
most selective to the alcohol products at all positions
entries 3 and 7) due to the bulkiness of the ligand com-
Compound 3 was
(
cifically, 1-octanol and 2-octanol, giving
a
total
pared to compounds 1 and 2 (entries 1, 2, 5 and 6). It was
observed that C(2) was the dominant position of attack,
as this carbon position is more susceptible to oxidation
compared to C(1), and according to thermodynamic cal-
culations (ΔG) this gives the most stable product.
Complexes (1–3) were also highly selective to alcohol
regioselectivity ratio of 1:5.6:3.9:4.2. The authors
suggested that the rigid pyridine backbone of the SNS
complexes hindered over-oxidation compared to the less
[
59]
rigid amine-based SNS compounds.
A different study
[
56]
showed that the C(2) position was the most activated car-
bon of the n-octane chain over Co PNP complexes bear-
ing a pentyl and iso-propyl moiety on the central
nitrogen donating atom. These complexes gave
products after workup at all carbon positions within
2
7 hr of reaction employing H O in the system (Table 5
2 2
and Figure S6). Consequently, the selectivity towards the
formation of ketones was very low. In a study by Pom-
ebeiro et al., homogeneous and immobilized Mn (salen)
complexes employing t-BuOOH as oxidant were investi-
gated. The authors noted no activation at the primary
carbon position and low regioselectivity profiles to the
regioselectivity
parameters
of 1:4.2:4.2:2.9 and
1:4.2:2.9:2.9, respectively with some activation at the C(1)
[
55]
position.
With the bidentate compound 4, the catalytic study
with t-BuOOH (entry 8, Table 4) shows a yield of 12% to
C8 products after 12 hr. The high catalytic activity shown
2 2
TABLE 5 Regioselectivity parameters C(1):C(2):C(3):C(4) in the oxidation of n-octane by t-BuOOH and H O over 1–4
a
b
Entry
Complex
Oxidant
Alcohol
Ketone
C(2):C(3):C(4)
1.8:2:1
Total
A/K
C(1):C(2):C(3):C(4)
1:13.5:6.5:7
0.3:13.5:6.5:7
1:21.4:10.7:12.9
0:1.9:1:1
C(1):C(2):C(3):C(4)
1:7.7:4.3:4
1
2
3
4
5
6
7
8
1
2
3
4
1
2
3
4
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
2.1:1.9:1
1.8:1.6:1
2:1.8:1
1:17.3:11.5:8.8
1:21.8:13.2:13
0:2:1.4:1
1.2:1
2:1
0.4:1
13:1
12:1
24:1
5:1
H
2
H
2
H
2
H
2
O
2
O
2
O
2
O
2
1:5:4:5
1.5:1.5:1
1.5:2:1
1:6.5:5.5:6
1:5.6:4.4:5.6
1:5.2:4.2:5.2
1:5.2:5:6.1
1:7.1:6.4:6.6
1:7.2:6.2:6.2
1:7:7:7.1
2:2:1
1.8:2:1
a
ꢁ
The total regioselective parameter accounts for all products (octanones, octanols, octanal and octanoic acid). All reactions were carried out at 80 C with a
compound loading of 1 mol% (0.008 mmol), an octane to oxidant ratio of 1:12 for both t-BuOOH (32 mmol) and H (42 mmol), cyclopentanone as the
2 2
O
internal standard (~10 mg) and reaction times of 12 and 27 hours for t-BuOOH and H
2
O
2
, respectively.
b
Alcohol: Ketone ratio.

NEW RU(II) PYRIDINE-BASED COMPLEXES IN THE PARTIAL OXIDATION OF OCTANE
11 of 13
SCHEME 1 Proposed mechanism for the
oxidation of n-octane catalysed by 1–4 with t-
BuOOH
by 4 is likely due to a less sterically hindered metal center
through loss of the benzene ring during the catalysis.
From this, a higher catalytic activity might be expected,
however, the phenyl substituted N-donor backbone, that
is largely electron withdrawing by the resonance effect,
also influences the activity. The cleavage of the benzene
ring, together with the lower steric effect of the bidentate
ligand, allows for enhanced access to the metal center,
which relates to the observed elevated production of
ketones (Figure 6A).
complex. This complex then provides an oxo-ruthenium
(IV) species, through homolytic cleavage of the O-O bond
in the alkylperoxo-ruthenium (II) complex. Abstraction
of a hydrogen atom from the hydrocarbon via the oxo-
3+
ruthenium (IV) species gives a Ru OHR• radical. The
catalytic cycle becomes complete upon transfer of the
hydroxy ligand to the R• radical which affords the alco-
2+
[60]
hol, ROH and the Ru (NNN) species.
Further oxida-
tion of the secondary alcohols results in the
corresponding ketones as observed over 4 with t-BuOOH.
In H O , a similar mechanism is proposed as outlined for
With H O , complex 4 was less active, giving a total
2
2
2 2
yield of 7% at the end of 27 hr (entry 16, Table 4). This
could be due to a lower concentration of OH• generated,
that essentially gives a low yield, when compared to the
pincer compounds. In addition, the low yield could be
further due to the electronic effect that stems from the
presence of the electron withdrawing phenyl moiety on
the nitrogen backbone of the compound. Looking at the
regioselective parameter C(1):C(2):C(3):C(4) over com-
pound 4, the lowest selectivity towards alcohols was
observed (entry 4, Table 5) with t-BuOOH. Again, C(2)
was the dominant position of attack, producing the most
stable products.
t-BuOOH (Scheme 1), but instead proceeds via an OH
radical to give the oxo-ruthenium (IV) species
(Scheme 2).
4 | CONCLUSION
A set of Ru pyridine-based compounds with aliphatic and
aromatic N-donor backbones was synthesized. Single
crystal XRD further confirmed the formation of pincer
complexes in the case of 1–3 and a bidentate complex for
4. The catalytic testing of complexes 1–4 using two oxi-
dants, tert-butylhydroperoxide and hydrogen peroxide,
and n-octane as a substrate, showed that with t-BuOOH,
the most active compounds were 2 and 4 with a yield of
12%. All complexes (except for 4 with t-BuOOH) gave
high selectivities to alcohols utilizing both oxidants.
Employing H O in the catalysis showed considerably
Adding the radical scavenger TEMPO to the reaction
mixture resulted in a drop in yield from 12% to 4% over
the [Ru{pyCH NC(CH ) CH py}C H ]/t-BuOOH system,
2
3 3
2
6
6
whilst no product yield was observed over 27 hours for
the [Ru{pyCH NC H CH py}C H ]/H O compound,
2
6
11
2
6
6
2 2
implying that a radical initiated mechanism is followed
when using both oxidants. Since alcohols are formed
dominantly by reaction of these compounds with n-
octane involving both oxidants (except for 4 with t-
BuOOH), consistent with the analytical method
employed, two kinds of mechanisms can be proposed for
the oxidation with H O or t-BuOOH catalyzed by Ru
2
2
improved activity for the pincer compounds in the oxida-
tion of n-octane with H O forming more alcohols com-
2
2
pared to t-BuOOH, which may be due to stabilization of
3+
the intermediate, Ru (NNN)-OH R•, or be responsible
for promoting OH• formation. Compound 3 showed
the highest activity, giving a 32% yield. 2-Octanol was
the dominant product observed over the Ru/t-BuOOH
2
2
2
(
Schemes 1 and 2).
+
In Scheme 1, the Ru (NNN) complex reacts with t-
and Ru/H O2 systems. Furthermore, 1-octanol was
2
BuOOH to produce an alkylperoxo-ruthenium (II)
also produced over the [Ru{pyCH NC H CH py}C H ]/
2
6
11
2
6
6

12 of 13
CHANERIKA ET AL.
t-BuOOH and [Ru{pyCH N(R)py}C H ]/H O systems.
Time-dependent studies reveal that over-oxidation is
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more prominent with t-BuOOH than with H O . Very
2
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good TONs were achieved with both oxidants, however,
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ACKNOWLEDGMENTS
[23] R. Chanerika, H. B. Friedrich, M. L. Shozi, Inorg. Chim. Acta.
We are grateful to the University of KwaZulu-Natal, the
Department of Science and Technology - National
Research Foundation Centre of Excellence in Catalysis,
c*change, (PAR 06.2) and the National Research Founda-
tion (Grant no. 111508) for financial assistance. The
authors gratefully acknowledge Dr Michael Nivendran
Pillay and Mr Sizwe Zamisa for x-ray crystallographic
data collection and refinement, and Dr. S. Alapour for a
valuable discussion.
https://doi.org/10.1016/j.ica.2019.118992
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How to cite this article: Chanerika R,
Friedrich HB, Shozi ML. Application of new Ru
(
II) pyridine-based complexes in the partial
oxidation of n-octane. Appl Organometal Chem.
019;e5361. https://doi.org/10.1002/aoc.5361
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