Utilisation of new NiSNS pincer complexes in paraffin oxidation

Article abstract of DOI:10.1016/j.ica.2018.04.033

Two series of closely related SNS pincer ligands (L) were synthesised with the major structural variation on the nitrogen backbone containing either the methyl [L = (RSCH₂CH₂)₂NMe: where R = Me (1), Et (2), Bu (3)] or the phenyl [L = (RSCH₂CH₂)₂NPh: where R = Me (4), Et (5), Cy (6)] functional group. When ligands 1–3 were complexed to Ni by reaction with Ni(DME)Cl₂ (DME = dimethoxyethane), they respectively yielded three new cationic dimeric [LNi(μ-Cl)₃NiL]⁺ complexes (7–9), whilst ligands 4–6 on reaction with Ni(PPh₃)₂Br₂ respectively yielded neutral mononuclear (LNiBr₂) complexes 10–12. All the new compounds were characterised by IR, HRMS, elemental analysis and in addition, single crystal X-ray diffraction for complexes 9–12. X-ray structural data of 9 revealed an unusual three chlorido-bridged Ni dimer with the SNS ligand coordinated in a facial binding mode to the two pseudo-octahedral Ni centres. Molecular structures of complexes 10, 11 and 12 each displayed five-coordinate distorted trigonal bipyramidal geometry around the nickel(II) metal centres. When utilised as catalysts in the tert-butyl hydroperoxide oxidation of n-octane, all the complexes showed activity to mainly products of internal carbon activation (octanones and secondary octanols) with 11 as the most active (10% total substrate to oxygenates yield), whereas 10 was the least active, but most selective towards alcohols (alcohol/ketone = 2.13).

Full text of DOI:10.1016/j.ica.2018.04.033

Inorganica Chimica Acta 479 (2018) 97–105
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Utilisation of new NiSNS pincer complexes in paraffin oxidation
⇑
⇑
Lynette Soobramoney, Muhammad D. Bala , Holger B. Friedrich
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban 4000, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Two series of closely related SNS pincer ligands (L) were synthesised with the major structural variation
Received 17 October 2017
Received in revised form 12 April 2018
Accepted 18 April 2018
on the nitrogen backbone containing either the methyl [L = (RSCH
3)] or the phenyl [L = (RSCH CH NPh: where R = Me (4), Et (5), Cy (6)] functional group. When ligands
–3 were complexed to Ni by reaction with Ni(DME)Cl (DME = dimethoxyethane), they respectively
NiL] complexes (7–9), whilst ligands 4–6 on reaction with
) complexes 10–12. All the new com-
2 2 2
CH ) NMe: where R = Me (1), Et (2), Bu
(
2
2 2
)
1
2
Available online 23 April 2018
+
yielded three new cationic dimeric [LNi(l-Cl)
3
3 2 2 2
Ni(PPh ) Br respectively yielded neutral mononuclear (LNiBr
Keywords:
pounds were characterised by IR, HRMS, elemental analysis and in addition, single crystal X-ray diffrac-
tion for complexes 9–12. X-ray structural data of 9 revealed an unusual three chlorido-bridged Ni dimer
with the SNS ligand coordinated in a facial binding mode to the two pseudo-octahedral Ni centres.
Molecular structures of complexes 10, 11 and 12 each displayed five-coordinate distorted trigonal bipyra-
midal geometry around the nickel(II) metal centres. When utilised as catalysts in the tert-butyl hydroper-
oxide oxidation of n-octane, all the complexes showed activity to mainly products of internal carbon
activation (octanones and secondary octanols) with 11 as the most active (10% total substrate to oxy-
genates yield), whereas 10 was the least active, but most selective towards alcohols (alcohol/ketone =
Homogeneous catalysis
SNS ligand
Nickel catalyst
n-alkane
Oxidation
2.13).
Ó 2018 Elsevier B.V. All rights reserved.
1
. Introduction
Exploration of the coordination chemistry and potential appli-
In a previous work [39] we reported new CoSNS complexes with
two types of known SNS ligands; [30,40–43] the first series were
characterised by a rigid backbone (with a central pyridine moiety
as the N-donor atom), while the second series contained a more
flexible backbone symmetrically built around an amine N-donor
atom. In both series, simple alkyl groups (respectively methylene
and ethylene) served as linkers to the S-donor atoms. When we
tested the complexes as catalysts in the oxidation of n-octane,
the results showed that those with the flexible backbone were
more active as catalysts. This observation prompted us to further
explore relatively flexible variants of the SNS ligands, i.e. those
constructed around a central amine N donor atom. For the current
studies, additional functionalities (alkyl and aryl groups) were
introduced on the central N-donor atom to allow for further mod-
ulation of its binding potentials to metal centres. A search of the
literature revealed a paucity of reports [44–46] on the types of
SNS ligands synthesized and reported in this work, hence many
of the compounds are reported for the first time. Herein, we
describe the synthetic protocols and characterisation of six SNS
ligands and their coordination to nickel to yield a total of six
new complexes. A complete study on their structural properties
and catalytic application in the oxidation of n-octane is also
reported and discussed.
cations of pincer compounds has become a dynamic area of
research over the last few decades [1–21]. In addition to the
greater stability brought about by the (tridentate) binding mode
of a pincer ligand, control over the electronic and steric properties
of their metal complexes is one of the desirable features that made
this type of ligands so popular [5,22–24]. More specifically, sulfur
and nitrogen based pincer ligands are considered hemilabile due
to dissimilarities in the donor strengths of the two coordinating
atoms i.e. the relatively soft sulfur donor combined with a hard
nitrogen donor. This feature is especially beneficial in catalysis as
it offers a balance between high reactivity and stability of the
metal complex [25]. Complexes bearing the SNS pincer moiety
have found applications in catalytic reactions such as ethylene
trimerization, [26–32] Suzuki coupling, [33] and transfer hydro-
genation, [34] as well as in medicinal applications where SNS pin-
cer complexes of radioactive metals were used as brain imaging
agents [35–38].
⇑
Corresponding authors.
E-mail addresses: bala@ukzn.ac.za
H.B. Friedrich).
(M.D.
Bala),
friedric@ukzn.ac.za
(
https://doi.org/10.1016/j.ica.2018.04.033
020-1693/Ó 2018 Elsevier B.V. All rights reserved.
0

9
8
L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
2
. Experimental section
2.4. Bis(methylthioethyl)methylamine (1)
2
.1. General
Sodium methanethiolate (3.38 g, 48.2 mmol) was added to a
solution of bis(2-chloroethyl)methylamine (2.64 g, 16.9 mmol) in
EtOH (50 ml) and the resultant mixture was refluxed for 2 h. The
solvent was removed in vacuo and the cream residue was purified
All reactions and manipulations were performed using standard
Schlenk techniques under nitrogen atmosphere. Solvents were
dried according to established methods, [47] and purged with high
purity nitrogen gas prior to use. Diethyl ether (Et
drofuran (THF) were dried over sodium wire and benzophenone,
absolute ethanol (EtOH) and methanol (MeOH) were dried over
magnesium turnings and iodine, while dichloromethane (DCM)
was dried over phosphorous pentoxide. Ni(DME)Cl
sized according to a procedure adapted from the literature, [48]
whilst Ni(PPh Br was purchased from Sigma. All other reagents
by elution with 5% MeOH in DCM through a silica column to yield a
1
2
O) and tetrahy-
pale yellow oil. Yield: 71%. H NMR (400 MHz, CDCl
3
): d 2.10 (s, 6H,
CH
-N). 13C APT
-S) neg, 31.78 (CH CH -N) pos,
CH
NMR (400 MHz, CDCl
42.03 (CH -N) neg, 56.95 (CH
for: C
3
-S), 2.27 (s, 3H, CH
): d 15.83 (CH
CH
3
-N), 2.62–2.56 (m, 8H, CH
2
2
3
3
2
2
3
2
2
-N) pos. HRMS ESI (m/z) Calcd
À1
2
was synthe-
7
H18NS
2
= 180.0881. Found: 180.0879. IR
m
max (cm ): 2956
(m), 2915 (s), 2843 (m), 2788 (m), 1456 (m), 1437 (m), 1110 (s),
699 (m).
3
)
2
2
were obtained commercially and used as received. All NMR spectra
were recorded using a Bruker Avance III 400 MHz spectrometer at
ambient temperature. The 1H NMR data are reported as chemical
2.5. Bis(ethylthioethyl)methylamine (2)
shift (d, ppm) and referenced to the solvent peak CDCl
attached proton test (APT) 13C NMR, which distinguishes between
quaternary C, CH, CH and CH carbons, are listed as chemical shift
d, ppm) and positive (pos) or negative (neg) with the correspond-
ing carbons in parentheses and referenced to the solvent peak
CDCl . The IR data were recorded on a Perkin Elmer Attenuated
Total Reflectance (ATR) spectrophotometer and elemental analyses
were performed on a Thermo-Scientific Flash 2000 CHNS/O ele-
mental analyzer, the HRMS was recorded on a Waters Micromass
LCT Premier TOF-MS, while the melting points were determined
using a Stuart Scientific melting point apparatus.
3
. The
Sodium metal (1.101 g, 47.9 mmol) and ethanethiol (3.6 ml,
7.9 mmol) were stirred together in EtOH (50 ml) under inert
4
2
3
atmosphere for 15 min in a 100 ml Schlenk flask. This mixture
was then transferred to a solution of bis(2-chloroethyl)methy-
lamine (2.49 g, 15.9 mmol) in EtOH (100 ml) via cannula and the
resultant solution was refluxed for 2 h whilst monitoring by TLC.
The solvent was then evacuated and the cream residue was puri-
(
3
fied analogously to 1 to obtain a pale yellow oil. Yield: 73%. 1
H
NMR (400 MHz, CDCl
s, 3H, CH -N), 2.53 (q, 4H, CH
N). C APT NMR (400 MHz, CDCl
3
): d 1.24 (t, 6H, CH
3
CH
2
-S, J = 7.4 Hz), 2.28
CH
(
3
3
CH -S), 2.63–2.59 (m, 8H, CH
2
2
2
-
13
3
): d 14.87 (CH
3
CH
2
-S) neg,
-N) neg,
2
5
2
2
6.21 (CH
7.41 (CH
3
CH
CH
2
-S) pos, 29.22 (CH
2
CH
2
-N) pos, 42.05 (CH
3
2
2
-N) pos. HRMS ESI (m/z) Calcd for: C
9
H22NS
2
=
2.2. Bis(2-chloroethyl)methylamine
À1
08.1194. Found: 209.1190. IR
871 (w), 2846 (w), 2790 (m), 1453 (s), 1109 (m), 733 (m).
m
max (cm ): 2961 (m), 2925 (s),
Into a 250 ml Schlenk flask containing 50 ml of DCM and 13.8
ml (120 mmol) of methyldiethanolamine, 26.0 ml (360 mmol) of
SOCl was added dropwise at 0 °C under vigorous stirring. The
2
2.6. Bis(butylthioethyl)methylamine (3)
resultant solution was stirred at room temperature for 2 h, after
which the solvent was removed in vacuo to yield a white solid
This ligand was synthesized and purified similarly to 2 with the
following masses and volumes: 2.493 g (16.0 mmol) of bis(2-chlor-
oethyl)methylamine, 1.102 g (47.9 mmol) of sodium metal and 5.1
ml (47.9 mmol) of butanethiol. The product was obtained as a yel-
1
which was dried under vacuum for several days. Yield: 98%.
NMR (400 MHz, CD OD): d 3.04 (s, 3H, CH -N), 3.69 (t, 4H, CH
N), 4.03 (t, 4H, CH CH OD): d
-N). 13C APT NMR (400 MHz, CD
8.14 (CH CH -N) pos, 41.44 (CH -N) neg, 58.22 (CH CH -N) pos.
Melting point = 106–107 °C.
H
3
3
2 2
CH -
2
2
3
1
3
2
2
3
2
2
low oil. Yield: 81%. H NMR (400 MHz, CDCl ): d 0.90 (t, 6H, CH -
3
3
CH CH CH -S, J = 7.3 Hz), 1.44–1.35 (m, 4H, CH CH CH CH -S),
2
2
2
3
2
2
2
1
4
.60–1.52 (m, 4H, CH
H, CH CH CH CH
3
CH
2
CH
2
CH
2
-S), 2.28 (s, 3H, CH
3
-N), 2.52 (t,
CH
-N). 13
CH -S) neg,
-N) pos, 31.87 (CH
-S) pos, 42.17 (CH -N)
3
2
2
2
-S, J = 7.3 Hz), 2.61 (br s, 8H, CH
): d 13.69 (CH CH CH
-S) pos, 29.67 (CH CH
-S) pos, 32.11 (CH CH CH CH
2
2
C
2.3. Bis(2-tosylethyl)phenylamine (Ph-Tos)
APT NMR (400 MHz, CDCl
2.00 (CH CH CH CH
CH CH CH
neg 57.44 (CH
3
3
2
2
2
2
3
2
2
2
2
2
3
-
In a 500 ml round bottom flask, phenyldiethanolamine (10.54 g,
8.2 mmol) was dissolved in 100 ml of THF. In a separate beaker,
3.92 g (598 mmol) of NaOH was dissolved in 100 ml of double-
2
2
2
3
2
2
2
3
5
2
2 2
CH -N) pos. HRMS ESI (m/z) Calcd for: C13
H30NS
2
À1
=
264.1820. Found: 264.1828. IR
m
max (cm ): 2956 (m), 2927
distilled water and added to the THF solution. A solution of tosyl
chloride (22.02 g, 116 mmol) in 100 ml of THF was added slowly
to the resultant mixture at 0 °C, which was then allowed to stir
at 0 °C for 30 min and thereafter at room temperature for a total
time of 4 h. The mixture was then poured into 200 ml of double-
distilled water and extracted with DCM (3 Â 100 ml). The com-
bined organic fractions were then washed with a saturated NaCl
(
m), 2872 (w), 2789 (w), 1458 (s), 1053 (m), 745 (m).
2.7. Bis(methylthioethyl)phenylamine (4)
Sodium methanethiolate (0.601 g, 8.58 mmol) was added to a
solution of bis(2-tosylethyl)phenylamine (1.389 g, 2.84 mmol) in
THF (50 ml) and the resultant mixture was left to reflux for 24 h.
The reaction mixture was filtered and the solvent was removed
4
solution, dried with MgSO and evacuated to yield a peach colored
oil which, upon standing overnight, turned into a solid. After
recrystallization from DCM and EtOH, a white crystalline product
1
in vacuo to yield a pale yellow oil. Yield: 71%. H NMR (400 MHz,
was obtained. Yield: 71%. 1H NMR (400 MHz, CDCl
): d 2.44 (s,
-N), 4.10 (t, 4H, CH CH -N),
CDCl
3.48 (t, 4H, CH
(t, 2H, Ph-N, J = 8.0 Hz). C APT NMR (400 MHz, CDCl
(CH -S) neg, 31.36 (CH CH -N) pos, 51.06 (CH CH -N) pos, 111.83
(Ph-N) neg, 116.65 (Ph-N) neg, 129.43 (Ph-N) neg, 146.79 (Ph-N)
3
3 3 2 2
): d 2.10 (s, 6H, CH -S), 2.62 (t, 4H, CH CH -N, J = 7.5 Hz),
6
6
7
H, CH
3
-tosyl), 3.56 (t, 4H, CH
2
CH
2
2
2
2
CH -N, J = 7.5 Hz), 6.59–6.65 (m, 3H, Ph-N), 7.16
2
1
3
.44 (d, 2H, aromatic), 6.72 (t, 1H, aromatic), 7.14 (t, 2H, aromatic),
3
): d 15.80
.28 (d, 4H, aromatic), 7.72 (d, 4H, aromatic). 13C APT NMR (400
3
2
2
2
2
3 3 2 2
MHz, CDCl ): d 21.65 (CH -tosyl) neg, 50.19 (CH CH -N) pos,
6
6.58 (CH CH
2
2
-N) pos, 112.03 (aromatic) neg, 117.61 (aromatic)
pos. HRMS ESI (m/z) Calcd for: C12
H
20NS
2
= 242.1037. Found:
À1
neg, 127.84 (aromatic) neg, 129.48 (aromatic) neg, 129.89 (aro-
242.1031. IR
m
max (cm ): 3092 (w), 3034 (w), 2958 (w), 2914
matic) neg, 132.62 (aromatic) pos. Melting point = 88–89 °C.
(m), 1597 (s), 1501 (s), 1350 (m), 1186 (m), 745 (s), 691 (s).

L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
99
2.8. Bis(ethylthioethyl)phenylamine (5)
(%) calc. for C18
50
H N
S
2 4
Cl
4
2
Ni O
4
: C, 28.98; H, 6.76; N, 3.75; found:
C, 28.48; H, 6.21; N, 3.31.
Sodium metal (0.194 g, 8.45 mmol) and ethanethiol (0.63 ml,
.45 mmol) were stirred together in EtOH (100 ml) under inert
8
2.12. Tri-l-chlorido-bis[bis(butylthioethyl)methylaminenickel(II)]
atmosphere for 15 min in a 250 ml Schlenk flask. This mixture
chloride (9)
was then transferred to a solution of bis(2-tosylethyl)phenylamine
(
1.378 g, 2.82 mmol) in THF (80 ml) via cannula at 0 °C and the
Complex 9 was synthesized according to the procedure
resultant solution was refluxed for 2 h whilst monitoring by TLC.
The mixture was then poured into 100 ml of double distilled water
and extracted three times with 50 ml portions of DCM. The organ-
ics were combined, washed with a brine solution and dried with
described for 7 with the following masses: 0.086 g (0.39 mmol)
of 3 and 0.074 g (0.39 mmol) of Ni(DME)Cl . Single crystals suitable
for analysis were obtained from the crude green oil. Yield: 74%.
2 2
Melting point: 89–91 °C. HRMS ESI (m/z) Calcd for: [(C13 ) -
2
H
29NS
max (cm ): 2958 (w),
2930 (m), 2871 (m), 1463 (s), 1426 (m), 1224 (m), 1043 (m), 736
(s). Anal. (%) calc. for C26 Cl Ni : C, 35.64; H, 7.82; N,
3.20; found: C, 35.54; H, 7.94; N, 2.85.
+
À1
MgSO
ation of the solvent. Yield: 73%. H NMR (400 MHz, CDCl
t, 6H, CH CH -S, J = 7.6 Hz), 2.62 (q, 4H, CH CH -S), 2.73 (t, 4H,
CH CH -N, J = 7.5 Hz), 3.54 (t, 4H, CH CH -N, J = 7.6 Hz), 6.71–
.65 (m, 3H, Ph-N), 7.24 (t, 2H, Ph-N, J = 8.1 Hz). C APT NMR
400 MHz, CDCl ): d 15.00 (CH CH -S) neg, 26.27 (CH CH -S) pos,
-N) pos, 51.51 (CH CH -N) pos, 111.80 (Ph-N) neg,
16.58 (Ph-N) neg, 129.54 (Ph-N) neg, 146.77 (Ph-N) pos. HRMS
4
. The product was obtained as a pale yellow oil upon evacu-
2 3
Ni Cl ] = 747.1255. Found: 747.1478. IR m
1
3
): d 1.29
(
3
2
3
2
H
68
N
S
2 4
4
2 5
O
2
2
2
2
13
6
(
3
3
2
3
2
2.13. Dibromido[bis(methylthioethyl)phenylamine]nickel(II) (10)
2
8.88 (CH
2
CH
2
2
2
1
A THF solution (3 ml) of ligand 4 (0.125 g, 0.519 mmol) was
ESI (m/z) Calcd for: C14
H
24NS
2
= 270.1350. Found: 270.1345. IR
3 2 2
added to a 3 ml solution of Ni(PPh ) Br (0.383 g, 0.515 mmol) in
À1
m
max (cm ): 3025 (w), 2962 (w), 2924 (w), 2869 (w), 1597 (s),
THF. The resultant green solution was left to stir at room temper-
ature over a period of four days. The solvent was then evacuated
and the green residue was washed with several 5 ml portions of
1
502 (s), 1349 (m), 1185 (m), 744 (s), 691 (s).
2
Et O. The product was then extract with 6 ml of DCM and upon
evacuation of the solvent a brown solid was obtain, identified as
the product. Single crystals were obtained from a DCM solution
2
.9. Bis(cyclohexylthioethyl)phenylamine (6)
This ligand was synthesized using the same method as for 5
2
of 10 layered with Et O. Yield: 26%. Decomposes > 200 °C. HRMS
with the following masses and volumes: 3.085 g (6.31 mmol) of
bis(2-tosylethyl)phenylamine, 0.432 g (18.92 mmol) of sodium
and 2.32 ml (18.92 mmol) of cyclohexanethiol. The product was
+
ESI (m/z) Calcd for: [C12
H19NS
2
BrNi] = 377.9496. Found:
À1
3
77.9503. IR
m
max (cm ): 3092 (w), 3024 (w), 2958 (w), 2914
obtained as a waxy solid. Yield: 84%. 1H NMR (400 MHz, CDCl
(w), 1597 (m), 1501 (s), 1350 (m), 745 (s), 691 (s). Anal. (%) calc.
for C12 NiBr : C, 31.34; H, 4.16; N, 3.05; found: C, 31.38; H,
.07; N, 2.87.
3
):
12-S), 1.99–1.93
-N, J = 7.5 Hz), 3.51 (t, 4H,
2
-N, J = 7.5 Hz), 6.70–6.65 (m, 3H, Ph-N), 7.23 (t, 2H, Ph-N,
H19NS
2
2
d 1.35–1.27 (m, 12H, C
m, 4H, C 12-S), 2.73 (t, 4H, CH
CH CH
J = 7.5 Hz). C APT NMR (400 MHz, CDCl
6
H
12-S), 1.79 (m, 4H, C
6
H
4
(
6
H
2
CH
2
2
1
3
2.14. Dibromido[bis(ethylthioethyl)phenylamine]nickel(II) (11)
3
): d 25.80 (C 12-S) pos,
6
H
2
4
1
6.12 (C
3.81(C
16.48 (Ph-N) neg, 129.51 (Ph-N) neg, 146.77 (Ph-N), pos. HRMS
6
H
12-S) pos, 27.28 (CH
2
CH
2
-N) pos, 33.90 (C H12-S) pos,
2
-N) pos, 111.83 (Ph-N) neg,
6
This complex was synthesized similarly to 10 except that DCM
was used as the solvent with the following masses: 0.146 g (0.543
6
H12-S) neg, 51.85 (CH
2
CH
mmol) of 5 and 0.394 g (0.530 mmol) of Ni(PPh
was obtained as a maroon crystalline solid. Single crystals were
grown from a DCM solution of 11 layered with Et O. Yield: 30%.
2
Melting point: 178–179 °C. HRMS ESI (m/z) Calcd for: [C14 -
3 2 2
) Br . The product
ESI (m/z) Calcd for: C22
H36NS
2
= 378.2289. Found: 378.2279. IR
À1
m
max (cm ): 3087 (w), 3058 (w), 3022 (w), 2923 (s), 2849 (m),
2
1
595 (m), 1503 (s), 745 (s).
H
23NS
max (cm ): 3032 (w),
963 (m), 2924 (m), 2863 (w), 1596 (m), 1588 (m), 1491 (s),
458 (s), 1315 (m), 779 (s), 727 (s), 707 (s), 579 (s), 496 (s). Anal.
NNiS : C, 34.46; H, 4.75; N, 2.87; found: C,
4.70; H, 4.85; N, 2.56.
+
À1
BrNi] = 405.9809. Found: 405.9814. IR
m
2.10. Tri- -chlorido-bis[bis(methylthioethyl)methylaminenickel(II)]
l
2
1
(
chloride (7)
%) calc. for C14
H23Br
2
2
An EtOH solution (3 ml) of ligand 1 (0.190 g, 1.06 mmol) was
added to a 5 ml solution of Ni(DME)Cl (0.201 g, 1.06 mmol) in
3
2
EtOH. The resultant green solution was left to stir overnight at
room temperature. The solvent was then evacuated and the green
2.15. Dibromido[bis(cyclohexylthioethyl)phenylamine]nickel(II) (12)
residue was stirred in 5 ml of Et
was washed several time with Et
Yield: 45%. Melting point: 253–255 °C. HRMS ESI (m/z) Calcd for:
2
O to obtain a green solid which
This complex was synthesized similarly to 11 with the follow-
2
O and collected by filtration.
ing masses: 0.103 g (0.273 mmol) of 6 and 0.192 g (0.258 mmol)
of Ni(PPh Br . The product was obtained as a purple crystalline
solid. Single crystals were grown from a DCM solution of 12 lay-
ered with Et O. Yield: 22%. Melting point: 190–191 °C. HRMS ESI
3
)
2
2
+
À1
[
2
1
(C
7
H
17NS
2
)
2
Ni
973 (w), 2922 (m), 2868 (m), 1463 (m), 1448 (m), 1420 (s),
251 (m), 1018 (m), 739 (s). Anal. (%) calc. for C14 Cl Ni
2 3
Cl ] = 578.94. Found: 578.95. IR mmax (cm ):
2
H
38
N
2
S
4
4
2
O
2
:
+
(
IR
7
m/z) Calcd for: [C22
H
35NS
max (cm ): 2925 (m), 2851 (m), 1446 (s), 1265 (m), 998 (m),
35NS NiBr : C, 44.32; H,
2
BrNi] = 514.0748. Found: 514.0740.
C, 25.71; H, 5.86; N, 4.28; found: C, 25.47; H, 5.37; N, 3.96.
À1
m
45 (m), 693 (m). Anal. (%) calc. for C22
H
2
2
2
.11. Tri- -chlorido-bis[bis(ethylthioethyl)methylaminenickel(II)]
l
5.92; N, 2.35; found: C, 44.31; H, 5.87; N, 2.55.
chloride (8)
2.16. Crystallographic analyses
Complex
8 was synthesized according to the procedure
described for 7 with the following masses: 0.165 g (0.80 mmol)
of 2 and 0.151 g (0.80 mmol) of Ni(DME)Cl . Yield: 74%. Melting
Single crystals for each complex were individually selected and
2
glued onto the tip of a glass fiber, mounted in a stream of cold
nitrogen and centered in the X-ray beam using a video camera. Sin-
gle-crystal X-ray diffraction data were collected on a Bruker KAPPA
APEX II DUO diffractometer using graphite-monochromated Mo-
+
point: 175–176 °C. HRMS ESI (m/z) Calcd for: [(C
9
H
21NS
2
)
2
Ni
2
Cl
3
]
À1
=
635.0000. Found: 635.0000. IR
m
max (cm ): 2964 (w), 2928 (m),
2
872 (m), 1450 (s), 1428 (m), 1259 (m), 1040 (m), 738 (m). Anal.

1
00
L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
K
a
radiation (
v
= 0.71073 Å). Data collection was carried out at
with the injector temperature set at 240 °C. Yield was calculated
based on the total moles of products formed divided by the initial
moles of substrate added into the reaction mixture and was
expressed as a percentage, while the selectivity was expressed as
the mole fraction of each product expressed as a percentage. All
catalytic reactions were performed in duplicate. Each reported data
was replicated within acceptable variance.
1
00(2) or 173(2) K. Temperature was controlled by an Oxford Cryo-
stream cooling system (Oxford Cryostat). Cell refinement and data
reduction were performed using the program SAINT [49]. The data
were scaled and absorption correction performed using SADABS
[
[
F
50]. The structures were solved by direct methods using SHELXS
50] and refined by full-matrix least-squares methods based on
2
using SHELXL [50] and using the graphics interface program X-
Seed [51,52]. All non-hydrogen atoms were refined anisotropically.
All hydrogen atoms were placed in idealised positions and refined
in riding models with Uiso assigned 1.2 or 1.5 times Ueq of their par-
ent atoms and the bond distances were constrained to 0.98 or 0.99
Å. Selected crystallographic and structural refinement data are
available as Electronic Supporting Information (ESI, Table S1). Crys-
tallographic data for the structures in this article have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC
3
. Results and discussion
3.1. Synthesis and characterization of ligands
This study reports on the synthesis and characterization of six
pincer-type SNS ligands, (the majority of which are new) and the
coordination chemistry of their Ni complexes. The SNS ligands pre-
pared contained a flexible, straight chained amine backbone func-
tionalised with a methyl or phenyl group as well as ethylene
linkers to the S-donor atoms. The substituents bonded to the S-
donor atoms were varied to investigate steric and electronic effects
on the complexes. The synthetic route followed for the preparation
of the ligands and complexes was developed and optimised to
obtain the best yield and purity of the compounds.
1
555285-1555288 for 9 – 12 respectively. These data can be
obtained free of charge at http://www.ccdc.cam.ac.uk/conts/re-
trieving.html (or from the Cambridge Crystallographic Data Centre,
1
2 Union Road Cambridge CB2 1EZ, UK; Fax: +44-1223/336-033; E-
mail: deposit@ccdc.cam.ac.uk).
2
.17. Paraffin oxidation studies
All the ligands (Scheme 1) were synthesized by the reaction of
bis(2-chloroethyl)methylamine or bis(2-tosylethyl)phenylamine
with the respective thiol, which was activated by stirring with
sodium metal in ethanol, to form the products as oils (with the
exception of 6) in 71–84% yields. The ligands were purified on a sil-
ica gel column and characterised by NMR, IR and HRMS.
NMR experiments ( H and C APT) were employed to elucidate
the structures of the ligands prepared, since these are reliable and
efficient techniques that provide important characterisation data.
Spectra obtained for each ligand showed clean signals which inte-
grated to the expected number of protons with no signs of impuri-
ties present. An analysis of the results revealed that the methyl
group bonded to the central N-donor atom on the ligand backbone
appeared as a singlet in the region of 2.27–2.28 ppm for 1–3. The
two symmetric ethylene linker groups gave rise to a multiplet or
a broad signal between 2.59 and 2.61 ppm which integrated to
the expected eight protons. In contrast, two triplets of four protons
each were observed for the ethylene spacers of the phenyl substi-
tuted ligands 4–6.
The catalytic reactions were carried out in 25 ml two-neck pear
shaped flasks fitted with a condenser and charged with acetonitrile
as the solvent, cyclopentanone as the internal standard, n-octane
as the paraffinic substrate and t-butyl hydroperoxide (TBHP) or
1
13
H
2
O
2
as the oxidant with the total volume of the reaction mixture
equating to 5 ml. The catalyst was introduced in the form of a stock
solution with the volumes varied so that the No. of moles of cata-
lyst remained constant at 9.56 x 10 mol. The catalyst to substrate
ratio was kept constant at 1:100 and the substrate to oxidant ratio
was varied in order to determine the optimum amount of oxidant
required for the reaction. The reactions were stirred in an oil bath
maintained at 50 °C for a period of 24 h after which a sample was
À6
removed, treated with an excess amount of PPh
was saturated) and filtered through a Celite plug. Thereafter, a 0.5
l aliquot was injected into the GC for analysis and quantification
3
(until the sample
l
of the products. The products were analysed using a PerkinElmer
Auto System gas chromatograph equipped with a flame ionisation
detector (FID) which was set at 260 °C. A 50 m  0.20 mm  0.5
mm Pona column was employed to efficiently separate the products
An HSQC NMR experiment (see Supplementary Information)
was undertaken to highlight the correlation between the broad sig-
2 2 5 2 3 2 2
Scheme 1. Synthetic route to the synthesis of ligands 1–6 and Ni complexes 7–12. (a): SOCl or TsCl, (b): Na/C H OH mixture, (c): Ni(DME)Cl and (d): Ni(PPh ) Br .

L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
101
nal of 2 and the related carbon atoms. It shows that all the associ-
ated eight protons are equivalent to each other and are in the same
environment, hence the overlapping signals. For the second series
of ligands functionalised by the phenyl N-backbone (4–6), the aro-
matic protons were all observed as expected further downfield in
the region of 6.61–7.24 ppm. The data obtained from IR and HRMS
analyses are all as expected for each of the prepared ligands.
this pattern was not observed for complexes 10–12 bearing the
phenyl N-substituent. The CHN analyses obtained for complexes
7–9 revealed the presence of varying amounts of water in the sam-
ple which was expected due to the hygroscopic nature of these
complexes. The data acquired for complexes 10–12 were within
the acceptable range which is indicative of their high purity.
3.3. Single crystal X-ray structural analysis of complexes
3.2. Synthesis and characterisation of complexes
Green single crystals were grown for complex 9 from a crude
The reaction of a slight molar excess of the respective SNS
green oil and for complexes 10–12 from DCM solutions layered
with diethyl ether to yield dark brown to purple crystals. Complex
9 was isolated as a bimetallic dimer with octahedral geometry
around each Ni(II) centre (Fig. 1), while complexes 10–12 (Figs. 2
and 3 respectively) crystallised as monomeric five coordinate com-
plexes that are characterised by trigonal bipyramidal geometry
around the metal.
ligands (L) {[L = (RSCH
3)] or [L = (RSCH CH
the metal salts (Ni(DME)Cl
tion of new cationic dimeric [LNi(
clear (LNiBr ) 10–12 complexes (Scheme 1). It is important to
2
CH
NPh: where R = Me (4), Et (5), Cy (6)]} with
or Ni(PPh Br ) resulted in the forma-
-Cl) NiL] (7–9) and mononu-
2 2
) NMe: where R = Me (1), Et (2), Bu
(
2
2 2
)
2
3
)
2
2
+
l
3
2
note that the relatively bulkier and rigid groups around the Ni(II)
centres in complexes 10–12 are responsible for the monomeric
nature of these complexes in comparison to the dimeric 7–9. Com-
plexes 10–12 bear planar phenyl substituents on the N-donor
atoms in comparison to the methyl N-substituents of 7–9 and in
addition, the former contain larger bromido ligands in comparison
to the chlorido-bearing 7–9. These reasons combine to explain the
structural differences between the two sets of NiSNS pincer com-
plexes. At the start of the reaction, an immediate colour change
was observed to indicate successful Ni complexation to the ligand.
Due to unresolved NMR signals, which we attributed to the param-
agnetic nature of nickel in the +2 state, we used other characterisa-
tion techniques. These included IR, MS, elemental analysis and
single crystal XRD for selected complexes, to analyse and unam-
biguously confirm the structural composition and purity of all
the new Ni complexes.
The prepared NiSNS complexes are hygroscopic and absorbed
moisture upon exposure to air. This was evident from the broad
band observed in the OÀH stretching region of their IR spectra.
Nevertheless, upon complexation, significant shifts in the frequen-
cies of bands associated with the SNS ligands were noted and inter-
preted as indicators of successful coordination of the isolated SNS
ligands to the Ni(II) centres. From the selected data listed in Table 1
Complex 9 crystallised in the triclinic P ꢀı space group as dimeric
species containing distorted octahedrally bonded Ni(II) centres
(Fig. 1) bridged via three chloride substituents. Related NiSNS com-
plexes with halogen bridging have been reported in the literature
[53,54]. The tridentate SNS ligand chelates in a facial binding mode
to the metal centre such that the S-donor atoms, Cl(1) and Cl(3) lie
in the same equatorial plane, while the N-donor atoms and Cl(2)
occupy axial positions of the octahedron. The coordination envi-
ronment around Ni(2) is identical to that of Ni(1).
The two octahedral metal centres are separated by a Ni∙∙∙Ni dis-
tance of 3.018 Å which is shorter than the sum of the van der
Waals radii for two nickel atoms (3.260 Å). Therefore, it can be
regarded as a relatively strong interaction, specifically in compar-
ison to the previously reported CoSNS complex [39] which dis-
played a Co∙∙∙Co distance of 3.684 Å and a related tetradentate Ni
complex containing an asymmetric pyrazolate ligand with a Ni∙∙∙Ni
distance of 3.823 Å [43]. Although it is quite rare to find Ni com-
plexes that are bridged via three chlorides, there are a few reports
on related complexes [55–57]. One such is reported by Wikstrom
et al., which contains a tridentate NNN ligand [57]. A Ni∙∙∙Ni dis-
tance of 3.086 Å was calculated for this complex, which is still
slightly longer than what is observed for 9. The crystal packing
between molecules of 9 displayed a head-to-head, tail-to-tail
arrangement of the crystal units with the head being the methy-
lene group and the tail referring to the butyl chains.
for complexes 7–9, it is clear that enhanced dp-pp overlap
between the nickel metal centre and the N-donor atom of the
SNS moiety occurred upon coordination with the consequence that
the ligand CÀN bond weakened while the alkyl CÀH stretch
strengthened, as shown by the IR results (see ESI).
ESI-MS data for all the metal complexes (7–12) displayed the
[
NiSNSX]+ ion (where X = Cl or Br) as the predominant species
and the specific isotopic patterns found were consistent and in line
with what is expected for halogenated complexes. Furthermore,
we observed for complexes 7–9, a mass corresponding to a dimer
linked by three chlorides. This suggested that these complexes pos-
sibly dimerise via three bridging chlorido groups. The observation
was later confirmed by X-ray crystallographic data of 9. However,
Table 1
Selected IR data for ligands (1–3) and the corresponding nickel complexes (7–9).
À1
Compound
IR mmax/cm
a
a
b
CÀH stretch
CÀH bend
CÀN stretch
1
7
2
8
3
9
2915
2922
2925
2930
2927
2930
1456
1464
1453
1449
1458
1464
1056
1048
1050
1040
1053
1043
Fig. 1. Molecular structure of 9 with hydrogen atoms omitted for clarity. Thermal
ellipsoids are drawn at the 50% probability level. Selected bond lengths (Å) and
angles (°): N(1)ÀNi(1), 2.135(2); S(1)ÀNi(1), 2.3857(8); S(2)ÀNi(1), 2.4168(7); Cl
(
1)ÀNi(1), 2.4303(7); Cl(2)ÀNi(1), 2.4159(7); Cl(3)ÀNi(1), 2.4562(7); S(1)ÀNi(1)ÀS
a
Strong.
Medium.
(2), 97.95(3); N(1)ÀNi(1)ÀS(1), 87.42(7); N(1)ÀNi(1)ÀS(2), 85.49(6); N(1)ÀNi(1)À
b
Cl(1), 97.11(6); N(1)ÀNi(1)ÀCl(2), 176.50(6); N(1)ÀNi(1)ÀCl(3), 94.06(7).

1
02
L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
Fig. 2. Molecular structures of 10 (I) and 11 (II) with hydrogen atoms omitted for clarity and thermal ellipsoids shown at the 50% probability level. Selected bond lengths (Å)
and angles (°) for 10: N(1)ÀNi(1), 2.106(2); S(1)ÀNi(1), 2.4319(8); S(2)ÀNi(1), 2.3981(8); Cl(1)ÀNi(1), 2.4481(4); Cl(2)ÀNi(1), 2.4147(4); S(1)ÀNi(1)ÀS(2), 170.13(3); N(1)À
Ni(1)ÀS(1), 85.07(7); N(1)ÀNi(1)ÀS(2), 85.06(7); N(1)ÀNi(1)ÀCl(1), 100.30(6); N(1)ÀNi(1)ÀCl(2), 143.78(6). For 11: N(1)ÀNi(1), 2.107(2); S(1)ÀNi(1), 2.4329(8); S(2)ÀNi(1),
2
.4310(7); Cl(1)ÀNi(1), 2.4313(4); Cl(2)ÀNi(1), 2.4221(4); S(1)ÀNi(1)ÀS(2), 166.49(3); N(1)ÀNi(1)ÀS(1), 83.27(7); N(1)ÀNi(1)ÀS(2), 84.87(7); N(1)ÀNi(1)ÀCl(1), 101.02(6);
N(1)ÀNi(1)ÀCl(2), 143.59(6).
Fig. 3. Molecular structure of 12 showing two crystallographically independent molecules in the asymmetric unit cell with hydrogen atoms omitted for clarity and thermal
ellipsoids shown at the 50% probability level. Selected bond lengths (Å) and angles (°): N(1)ÀNi(1), 2.114(2); S(1)ÀNi(1), 2.4283(9); S(2)ÀNi(1), 2.4291(10); Cl(1)ÀNi(1),
2
1
.4461(5); Cl(2)ÀNi(1), 2.4293(5); S(1)ÀNi(1)ÀS(2), 158.93(3); N(1)ÀNi(1)ÀS(1), 84.09(8); N(1)ÀNi(1)ÀS(2), 82.97(8); N(1)ÀNi(1)ÀCl(1), 97.44(7); N(1)ÀNi(1)ÀCl(2),
48.72(8).
Complexes 10–12 all crystallised in the P2
1
space group of the
158.93(3)°]. This can be attributed to an increased steric crowding
around the metal centre resulting in a decreased ‘bite angle’.
An observation of the crystal packing of 10–12 revealed a sim-
ilar ordered arrangement of alternating units forming parallel
sheets in a head-to-tail configuration, where the phenyl and bro-
mide groups define the head and tail of the molecule respectively.
Intriguingly, the cyclohexyl groups present in 12 do not hinder the
packing efficiency of the molecule in the crystal lattice, which
resulted in a more closely packed arrangement as compared to
10 and 11 (See the ESI for packing Figures).
monoclinic crystal system. Only 12 crystallised with two identical
but independent molecules in the asymmetric unit cell as shown in
Fig. 3. All three complexes exhibit a five coordinate, distorted trig-
onal bipyramidal geometry around the metal with the S-donor
atoms occupying axial positions and the equatorial sites taken up
by the N-donor and the two bromide substituents.
The phenyl ring, N(1), Ni(1), Br(1) and Br(2) reside in the same
plane for complexes 10 and 11 (Fig. 2) with this also serving as a
plane of symmetry in 10. However, for 12, the phenyl ring deviates
from the [N(1), Ni(1), Br(1), Br(2)] plane by 0.026–0.500 Å. The
general trend observed is that as the flexibility of the substituents
on the S-donor atoms increased, the degree of asymmetry of the
entire complex also increased. Furthermore, the ‘bite angle’ defined
by the S(1)ÀNi(1)ÀS(2) chelation gradually became more acute as
the substituents (R) on the S-donor atoms increased in steric com-
plexity [methyl = 170.13(3), ethyl = 166.49(3) and cyclohexyl =
3.4. Application of the NiSNS complexes in n-octane oxidation
The prepared nickel complexes were applied as catalysts in the
oxidation of n-octane with both H
(TBHP) as the source of oxygen in acetonitrile medium. Tests using
as the oxidant gave yields equivalent to that of the blank
2 2
O and tert-butyl hydroperoxide
2 2
H O

L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
103
reaction (absence of catalyst) over 24 h (1–2%), which indicate that
the catalysts were inactive for CAH functionalisation with this par-
ticular oxidant. A colour change from green to colourless was also
stituted with ethyl groups on the S-donor atoms, gave a signifi-
cantly higher yield of 10% at 48 h. This suggests that electronic
effects and increased hydrophobicity dominate the catalytic activ-
ity, since the ethyl groups are better electron donors and more
hydrophobic than the methyl substituents. Although catalyst 12
is electronically the richest in the series due to the cyclohexyl S-
substituents, in comparison to 11, a lower yield was recorded indi-
cating that both electronic and steric effects contribute to the cat-
alytic activity. The cyclohexyl rings contributed to a degree of
steric crowding around the active site which slightly decreased
its activity.
Product selectivity is an important aspect in determining the
efficiency of a catalytic system. Obtaining a single product would
be ideal but also unrealistic especially in the oxidation of n-octane.
Considering the product distribution pattern obtained for both cat-
alyst series, a mixture of octanols and octanones oxygenated at
carbon positions 2, 3 and 4 (C-2, C-3 and C-4) of the octane C-8
backbone was observed. No terminal carbon (C-1) activation was
recorded for any of the systems in this study. The combined alcohol
and ketone selectivities for 7 are presented in Fig. 5, from which
the following points are observed:
observed upon the addition of H
decomposition. Therefore, TBHP was investigated further, since it
is a milder oxidant in comparison to H
2 2
O , which is indicative of catalyst
2 2
O .
Preliminary studies established the optimum conditions for the
oxidation reaction as 50 °C, at a catalyst to octane to TBHP ratio of
1
:100:1200 and a reaction time of 48 h (data was also recorded at
2
4 h). The cost and efficiency of utilising the oxidant is critical in
the successful adoption of any oxidation catalyst system, hence
the octane to TBHP ratio was thoroughly investigated (1:3, 1:6,
1
:9, 1:12, 1:15 and 1:18), however, the best balance between yield
and product selectivity was observed at 1:12 substrate to oxidant
ratio. A blank reaction gave a yield of 2% in 48 h, while reactions
with the simple salts (NiCl and NiBr ) gave yields of 3 and 5%
2 2
respectively. Catalytic data highlighting yields to total oxidation
products for all the complexes is presented in Fig. 4.
The yields were calculated after treatment of the reaction ali-
quot with PPh , in order to reduce any octyl hydroperoxides pre-
3
sent in the sample. This is a standard and largely accepted
procedure for paraffin oxidation reactions first developed and
reported by the Shul’pin group [58].
For catalysts 7–9, bearing the sterically smaller N-substituent, it
is clear that the yields are substantially higher than that over the
ꢀ Octanones represent the majority product class at both 24 and
48 h reaction periods.
ꢀ Accumulation of the ketones seems to be a secondary reaction
of alcohol over-oxidation. This is clear from the increased
ketone selectivity at 48 h which is matched by decreased alco-
hol selectivity.
2
metal precursor (NiCl ). This suggests that the ligands influenced
the enhanced activities of the catalysts. Furthermore, a slight
increase in yield was observed on extending the time of reaction
from 24 to 48 h, with 7 giving the highest yield of 9% at 48 h.
The ligand of this catalyst contains the smallest R-substituent
It is also important to note that a similar trend is observed for
the rest of the catalysts, indicating that they all followed a similar
reaction pathway.
3
bonded to the S-donor atoms (-CH ) which resulted in the least
sterically hindered metal centre. The metal centre is the active site
to which the substrate binds, hence in theory, a more sterically
available metal centre should render higher activity than a con-
gested one that offers constrained access to the substrate n-octane.
This indicates the role of the ligand in determining the availability
of binding sites for the substrate, which ultimately determines cat-
alyst efficiency. The yield then drops to 7% for 8, presumably
because of an increase in the steric crowding around the metal cen-
tre caused by the S-donor ethyl substituent. However, the slight
increase to 8% for catalyst 9 can be attributed to an electronic effect
caused by the electron donating long chain butyl groups.
The catalytic activities of the more rigid phenyl N-substituted
catalysts showed a different trend to those of the methyl N-substi-
tuted series. Catalyst 10, containing the methyl substituents on the
S-donor atoms was the least active giving a yield only marginally
higher than that of the metal precursor at 48 h. However, 11, sub-
The product distribution pattern, which displays the individual
product selectivities as well as the regioselectivity parameters at
each carbon, i.e. C(2):C(3):C(4) is presented in Table 2. Amongst
the methyl N-substituted catalysts, 7 and 9 showed higher selec-
tivity to the ketone products with low alcohol/ketone (A/K) ratios
of 0.28 and 0.30 respectively. Overall, catalyst 10 is the most selec-
tive to the alcohols with the highest A/K ratio of 2.13, presumably
because of its comparatively lower activity. Hence, its ability to
further oxidize the initial alcohol products to ketones was also
low, meaning that at the end of the reaction period, more of the
alcohol products remain. A low A/K ratio of 0.35 was observed
for 11 and this can be attributed to the higher activity it displayed,
which further emphasizes our belief that over-oxidation, driven by
a very active catalyst, is responsible for the prevalence of ketone
products in this systems.
Fig. 4. Yields obtained for the catalysts bearing the methyl N-substituent (7–9)
and phenyl N-substituent (10–12). Conditions: octane:TBHP = 1:12, Temp = 50 °C,
Time = 24 and 48 h.
Fig. 5. Selectivities to combined alcohol and ketone products for catalyst 7.
Conditions: Octane:TBHP = 1:12, Temp = 50 °C, Time = 24 and 48 h.

1
04
L. Soobramoney et al. / Inorganica Chimica Acta 479 (2018) 97–105
Table 2
Product selectivity and regioselectivity parameters in the oxidation of n-octane.a
Product selectivity (%)
A/Kc
Catalyst
2-ol
3-ol
4-ol
2-one
3-one
4-one
C(2):C(3):C(4)b
7
8
9
1
1
1
10
26
11
34
13
18
31
23
6
12
6
17
7
9
6
12
6
17
6
9
28
18
29
10
28
23
14
20
24
15
22
10
21
18
13
17
26
17
26
12
25
23
15
20
1.3:1:1.1
1.5:1:1
0.28
1.00
0.30
2.13
0.35
0.56
1.38
0.75
1.5:1:1.2
1.7:1:1.1
1.4:1:1.1
1.6:1:1.2
1.7:1:1
0
1
2
NiCl
NiBr
2
14
10
13
10
2
1.6:1:1.1
a
Reaction conditions: catalyst (9.56 Â 10 mol, 1.91 Â 10À3 M), n-octane (9.56 Â 10À4 mol, 0.155 ml), TBHP (70% in H
À6
2
O, 0.0164 mol), 50 °C, 48 h, MeCN added as a
solvent to give a total reaction volume of 5 ml.
b
Total regioselectivity parameter takes into consideration both alcohol and ketone products at each carbon position and these values are normalized, i.e. taking into
account the number of hydrogen atoms, 3 for the terminal carbon (CH ) and 2 for each of the unique internal carbon (CH ) positions.
A/K ratio gives the relative selectivity irrespective of carbon position of all alcohol/all ketone products, i.e. total moles of octanols divided by total moles of octanones.
3
2
c
In line with literature reports, the total regioselectivity param-
eter indicated that the internal C(2) hydrogens of the n-octane
chain were the most reactive [59–61]. Results obtained in this
study are comparable to those reported by Pombeiro and co-work-
ers using a Mn(salen) catalyst with TBHP [59]. They had also
observed no C(1) activation and reported similar regioselectivity
numbers for the oxidation of n-octane.
References
[
[
[
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tion was conducted under our standard conditions with catalyst 11
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the most prominent position of attack for all the catalytic systems
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