Flexible SNS pincer complexes of copper: Synthesis, structural characterisation and application in n-octane oxidation

Article abstract of DOI:10.1016/j.poly.2019.02.016

Two new tridentate SNS ligands based on a flexible, straight chain amine backbone were prepared and in combination with three related but previously reported ligands were reacted with CuCl₂ to yield new tridentate pincer-type copper(II) complexes. The molecular structures of the new ligands bis(cyclohexylthioethyl)methylamine (L1d), bis(t-butylthioethyl)methylamine (L1e), and all the complexes Cu[bis(Rthioethyl)methylamine]Cl₂ (1) where R = methyl (a); ethyl (b); butyl (c); cyclohexyl (d) and t-butyl (e); were confirmed by a range of spectroscopic and analytical techniques. Single crystal X-ray diffraction analysis determined the solid-state structures of the salt of L1e and the complexes. Complexes 1a-e all exhibited five-coordinate ligand geometry around each Cu(II) centre defined by the tridentate SNS and two chlorido donors. The complexes were applied as catalysts in the oxidation of n-octane using t-butyl hydroperoxide and hydrogen peroxide (H₂O₂) as oxidants. The H₂O₂ based systems yielded up to 36% conversion of n-octane to C-8 oxygenates with very high alcohol selectivity of up to 78%.

Full text of DOI:10.1016/j.poly.2019.02.016

Accepted Manuscript
Flexible SNS pincer complexes of copper: synthesis, structural characterisation
and application in n-octane oxidation
Lynette Soobramoney, Muhammad D. Bala, Holger B. Friedrich, Michael N.
Pillay
PII:
S0277-5387(19)30115-9
https://doi.org/10.1016/j.poly.2019.02.016
POLY 13760
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
6 November 2018
9 February 2019
Please cite this article as: L. Soobramoney, M.D. Bala, H.B. Friedrich, M.N. Pillay, Flexible SNS pincer complexes
of copper: synthesis, structural characterisation and application in n-octane oxidation, Polyhedron (2019), doi:
https://doi.org/10.1016/j.poly.2019.02.016
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Flexible SNS pincer complexes of copper: synthesis, structural
characterisation and application in n-octane oxidation
Lynette Soobramoney[a], Muhammad D. Bala[a], Holger B. Friedrich[a] and Michael N. Pillay[a]
Abstract: Two new tridentate SNS ligands based on a flexible, straight chain amine
backbone were prepared and in combination with three related but previously reported ligands
were reacted with CuCl2 to yield new tridentate pincer-type copper(II) complexes. The molecular
structures of the new ligands bis(cyclohexylthioethyl)methylamine (L1d), bis(t-
butylthioethyl)methylamine (L1e), and all the complexes Cu[bis(Rthioethyl)methylamine]Cl2 (1)
where R = methyl (a); ethyl (b); butyl (c); cyclohexyl (d) and t-butyl (e); were confirmed by a
range of spectroscopic and analytical techniques. Single crystal X-ray diffraction analysis
determined the solid-state structures of the salt of L1e and the complexes. Complexes 1a-e all
exhibited five-coordinate ligand geometry around each Cu(II) centre defined by the tridentate
SNS and two chlorido donors. The complexes were applied as catalysts in the oxidation of n-
octane using t-butyl hydroperoxide and hydrogen peroxide (H2O2) as oxidants. The H2O2 based
systems yielded up to 36% conversion of n-octane to C-8 oxygenates with very high alcohol
selectivity of up to 78%.
Keywords: Pincer ligands; SNS donors; Copper catalysis, Octane oxidation.
[a]
School of Chemistry and Physics,
University of KwaZulu-Natal,
Private Bag X54001, Durban 4000,
South Africa
Email: bala@ukzn.ac.za
1

1.
Introduction
Pincer complexes containing SNS ligand frameworks have received relatively less attention in
comparison to other pincers like the popular PNP donor set. However, since the discovery of
SNS complexes of chromium1-6 as useful ethylene trimerisation catalysts, research exploring the
SNS pincer backbone has advanced. To date, SNS pincer complexes of Cr(III),1 Mo(III),7
W(IV),8 Ru(II),9-11 Co(II),12 Ir(I),13 Ni(II),14 Pd(II),15,16 Pt(II),17,18 Cu(II),19 Zn(II),20-22 Cd(II),23
and Hg(II)23 have been successfully synthesised and characterised. The synthesis of these
compounds is generally straightforward and does not usually involve rigorous inert conditions as
required for air sensitive phosphine-based tridentate ligands. Another added advantage of the
tridentate SNS donor set is the mixed donor strength (soft S- and hard N-donors) of the
backbone, which renders the ligands hemilabile, an attractive feature that may be exploited in
catalytic applications of resulting complexes.
There are many reports on the application of copper(II) complexes containing SNS ligands as
catalysts in cycloaddition reactions19,24 as well as in the polymerisation of methyl
methacrylate.25,26 In an earlier study,12 we reported on cobalt(II) complexes containing pincer-
type SNS ligands with a linear straight chain amine and a constrained six-membered pyridine
ring as the N-donors of the ligands. In that study, the flexibility of the ligand backbone was
observed to be the key driver of catalyst activity such that the amine-based complexes proved to
be the more efficient catalysts with significantly higher substrate conversions when compared to
the more rigid pyridine-based variants. However, due to the unconstrained geometric disposition
of the elements around the pincer backbone, the amine-based SNS ligands also yielded
complexes that were unstable and challenging to handle.
Hence, in this study, we present a series of SNS pincer complexes of copper(II) with a modified
amine N-donor backbone, aimed at circumventing the instability concerns of the previous Co(II)
series of complexes combined with the well-established catalytic abilities of Cu2+ ions in
oxidation reactions.27-30 Therefore, we herein present an exploration on the relationship between
ligand backbone flexibility and catalytic activity of Cu(II) complexes containing pincer SNS
ligands.
2

2.
Experimental Section
2.1 General
An inert nitrogen atmosphere was employed for the synthesis of the ligands and complexes. All
solvents were dried according to standard methods31 and dissolved oxygen was purged with high
purity nitrogen gas prior to use. Diethyl ether (Et2O) and tetrahydrofuran (THF) were dried over
sodium wire and benzophenone, absolute ethanol (EtOH) and methanol (MeOH) were dried over
magnesium turnings and iodine, and dichloromethane (DCM) was dried over phosphorous
pentoxide. All other reagents were purchased commercially and used as received. All NMR
spectra were recorded using a Bruker AvanceIII 400 MHz spectrometer at ambient temperature.
The 1H NMR data are reported as chemical shift (δ, ppm) and referenced to the solvent peak
CDCl3. The proton decoupled 13C NMR data are presented as chemical shift (δ, ppm) and
referenced to the solvent peak CDCl3 with the specific carbon indicated in parentheses. The 13C
APT NMR data, which distinguishes between quaternary C, CH, CH2 and CH3 carbons, are listed
as chemical shift (δ, ppm) and positive (pos) or negative (neg) with the corresponding carbons in
parentheses. The IR spectra were recorded on a Perkin Elmer attenuated total reflectance (ATR)
spectrophotometer and elemental analyses were performed on a Thermo-Scientific Flash 2000
CHNS/O elemental 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. Recently, we communicated the synthetic procedures and full characterisation of
ligands L1a-c (Scheme 1).32
2.1.1 Synthesis and characterisation of new ligands and complexes
bis(Cyclohexylthioethyl)methylamine (L1d). This ligand was synthesized according to a
procedure we previously reported,32 with the following masses and volumes: 3.454 g (22.1
mmol) of bis(2-chloroethyl)methylamine, 1.527 g (66.4 mmol) of sodium metal and 8.1 ml (66.4
mmol) of cyclohexanethiol. The cream residue obtained after evacuation of the solvent was
purified on a silica column, first with hexane to remove excess starting material and then with
3

5% MeOH in DCM to elute the product, which was isolated as a pale yellow oil. Yield: 84%. 1H
NMR (400 MHz, CDCl3): δ 1.19-1.31 (m, 12H, C6H12-S), 1.73 (m, 4H, C6H12-S), 1.93 (m, 4H,
C6H12-S), 2.25 (s, 3H, CH3-N), 2.55-2.63 (m, 10H, C6H12-S, CH2CH2-N). 13C APT NMR (100
MHz, CDCl3): δ 25.83 (C6H12-S) pos, 26.09 (C6H12-S) pos, 27.68 (CH2CH2-N) pos, 33.78
(C6H12-S) pos, 42.09 (C6H12-S) neg, 43.72 (CH3-N) neg, 57.76 (CH2CH2-N) pos. HRMS ESI
(m/z) Calcd for: C17H33NS2Na = 338.1952. Found: 338.1949. IR νmax (cm-1): 2924 (s), 2850 (m),
2788 (w), 1447 (s), 999 (m), 745 (w).
bis(t-Butylthioethyl)methylamine (L1e). This ligand was synthesized and purified similarly
following our method,32 with the following masses and volumes: 1.163 g (7.5 mmol) of bis(2-
chloroethyl)methylamine, 0.539 g (23.4 mmol) of sodium metal and 2.6 ml (23.4 mmol) of tert-
butylthiol. The product was isolated as an orange oil. Yield: 80%. 1H NMR (400 MHz, CDCl3):
δ 1.33 (s, 18H, C4H9-S), 2.32 (s, 3H, CH3-N), 2.63 (m, 8H, CH2CH2-N). 13C APT NMR (100
MHz, CDCl3): δ 26.00 (CH2CH2-N) pos, 31.01 (C4H9-S) neg, 41.97 (C4H9-S) pos, 42.07 (CH3-
N) neg, 57.53 (CH2CH2-N) pos. HRMS ESI (m/z) Calcd for: C13H29NS2Na = 286.1639. Found:
286.1637. IR νmax (cm-1): 2959 (s), 2899 (w), 2862 (w), 2790 (w), 1458 (s), 1363 (s), 1162 (s),
1055 (m), 750 (w).
Cu[bis(methylthioethyl)methylamine] (1a). A solution of L1a (0.1759 g, 0.98 mmol) in 4 ml
of MeOH was added to a light green MeOH solution (4 ml) of CuCl2∙2H2O (0.1672 g, 0.98
mmol). Immediately, a colour change from light to dark green was observed and a green
precipitate began to form. The reaction mixture was allowed to stir at room temperature for 2 h,
after which the volume was reduced to ~2 ml and filtered via cannula. The precipitate was
washed several times with 3 ml portions of Et2O and dried to yield a green microcrystalline
solid. Single crystals suitable for X-ray diffraction were grown from the mother liquor. Yield:
71%. Melting point: 133-134 °C. HRMS ESI (m/z) Calcd for: C7H17NS2Cu = 242.0098. Found =
242.0094. IR νmax (cm-1): 2991 (w), 2963 (w), 2916 (m), 2857 (w), 2808 (w), 1472 (m), 1441 (s),
1424 (s), 1231 (m), 1085 (m), 741 (s). Anal. (%) calc. for C7H17NS2CuCl2: C, 26.8; H, 5.5; N,
4.5; found: C, 26.8; H, 5.6; N, 4.5.
Cu[bis(ethylthioethyl)methylamine] (1b). This complex was synthesized analogously to 1a
with the following masses: 0.142 g (0.69 mmol) of L1b and 0.1167 g (0.69 mmol) of
CuCl2∙2H2O. A colour change from light to dark green was observed, however no precipitate had
4

formed, and therefore the reaction was left to stir overnight at room temperature. The solution
was concentrated and a green oil crashed out upon the addition of Et2O. After decanting the
solvent, the oil was extracted with DCM and hexane. Upon evacuation of the supernatant, a dark
green crystalline solid was obtained. Single crystals suitable for analysis were grown by slow
diffusion of Et2O into a concentrated MeOH solution of 1b. Yield: 64%. Melting point: 118-119
°C. HRMS ESI (m/z) Calcd for: C9H21NS2Cu = 270.0411. Found = 269.9984. IR νmax (cm-1):
2961 (w), 2924 (w), 2866 (m), 1472 (w), 1446 (s), 1423 (m), 1380 (m), 1231 (m), 1086 (m), 735
(s). Anal. (%) calc. for C9H21NS2CuCl2: C, 31.6; H, 6.2; N, 4.1; found: C, 31.6; H, 6.3; N, 4.1.
Cu[bis(butylthioethyl)methylamine] (1c). A similar procedure as 1b was followed to obtain 1c
with the following masses: 0.232 g (0.88 mmol) of L1c and 0.147 g (0.86 mmol) of CuCl2∙2H2O.
An analogous colour change as the previous reactions was observed and the reaction solution
was evacuated to dryness after stirring overnight. The green residue was washed with Et2O,
taken up in 3 ml of DCM and the product crashed out as a green powder upon the addition of 2
ml of Et2O. The filtrate was removed by cannula and the solid was washed several times with
Et2O and dried under vacuum to afford a bright green powder. Single crystals suitable for
analysis were grown from a solution of 1c in DCM layered with Et2O. Yield: 81%. Melting
point: 101-103 °C. HRMS ESI (m/z) Calcd for: C13H29NS2Cu = 326.1037. Found = 326.0524. IR
νmax (cm-1): 2958 (w), 2930 (m), 2871 (m), 1463 (s), 1426 (m), 1224 (m), 1043 (m), 736 (s).
Anal. (%) calc. for C13H29NS2CuCl2: C, 39.2; H, 7.4; N, 3.5; found: C, 39.2; H, 7.6; N, 3.6.
Cu[bis(cyclohexylthioethyl)methylamine] (1d). A similar procedure as complex 1b was
adapted to synthesize complex 1d with the following masses: 0.210 g (0.67 mmol) of L1d and
0.112 g (0.66 mmol) of CuCl2∙2H2O. The solution was concentrated to 1 ml and stirred in 10 ml
of Et2O overnight. A dark green solid was obtained, which was washed with several portions of
Et2O and dried in vacuo. Single crystals were grown from a MeOH solution layered with
heptane. Yield: 92%. Melting point: 120-121 °C. HRMS ESI (m/z) Calcd for: C17H33NS2Cu =
378.1350.
Found = 378.0914. IR νmax (cm-1): 2922 (s), 2850 (m), 1447 (s), 1265 (m), 1042 (w), 732 (m).
Anal. (%) calc. for C17H33NS2CuCl2: C, 45.4; H, 7.4; N, 3.1; found: C, 45.4; H, 7.3; N, 2.8.
Cu[bis(t-butylthioethyl)methylamine] (1e). A similar procedure as 1a was adapted to
synthesize 1e with the following masses: 0.140 g (0.53 mmol) of L1e and 0.090 g (0.53 mmol)
5

of CuCl2∙2H2O. A precipitate formed within a few minutes, however the reaction mixture was
allowed to stir at room temperature for a further 2 h, after which the mixture was concentrated to
~3 ml and filtered via cannula. The precipitate was washed several times with 3 ml portions of
Et2O and dried to yield a lime green powder. Single crystals suitable for X-ray diffraction were
grown from a DCM solution of 1e layered with Et2O. Yield: 73%. Melting point: 118-119 °C.
HRMS ESI (m/z) Calcd for: C13H29NS2Cu = 326.1037. Found = 326.1037. IR νmax (cm-1): 2956
(w), 2862 (m), 1461 (m), 1363 (m), 1209 (m), 1163 (s), 998 (m), 745 (m). Anal. (%) calc. for
C13H29NS2CuCl2: C, 39.2; H, 7.4; N, 3.5; found: C, 39.0; H, 7.3; N, 3.2.
2.1.2 Crystallographic analyses
Single-crystal X-ray diffraction data were collected on a Bruker KAPPA APEX II DUO
diffractometer using graphite-monochromated Mo-Kα radiation ( = 0.71073 Å). Data collection
was carried out at 296(2), 173(2) or 100(2) K. Temperature was controlled by an Oxford
Cryostream cooling system (Oxford Cryostat). Cell refinement and data reduction were
performed using the program SAINT.33 The data were scaled and absorption correction
performed using SADABS.34 The structure was solved by direct methods using SHELXS34 and
refined by full-matrix least-squares methods based on F2 using SHELXL34 and using the graphics
interface program X-Seed35,36. 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 parent atoms with C-H bond distances constrained to 0.98
or 0.99 Å. Molecular structures and packing diagrams were generated using the program Olex2,
version 1.2.37 Crystallographic data for the structures in this article have been deposited with the
Cambridge Crystallographic Data Centre, with numbers CCDC 1831809 (L1e), 1831810 (1a) ,
1831814 (1b), 1831811 (1c), 1831817 (1d) and 1831812 (1e). These data can be obtained free of
charge at http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12 Union Road Cambridge CB2 1EZ, UK; Fax: +44-1223/336-
033; E-mail: deposit@ccdc.cam.ac.uk).
2.1.3 Alkane oxidation studies
6

A PerkinElmer Auto System gas chromatograph was utilised to analyse the products, which was
equipped with a flame ionisation detector (FID) set at 260 °C. A Pona column (50 m x 0.20 mm
x 0.5µm) was employed for the separation of the products with the injector temperature set at
240 °C. All catalytic reactions were carried out in duplicate in 25 ml two-neck pear shaped flasks
into which the reaction components were added and connected to a reflux condenser.
Acetonitrile was employed as the solvent while cyclopentanone and n-octane was used as the
internal standard and paraffin substrate respectively.
Two different types of oxidants were investigated i.e. t-butyl hydroperoxide (TBHP) and
hydrogen peroxide (H2O2). Furthermore, the total volume of the reaction mixture was kept
constant at 5 ml. The catalyst was introduced in the form of a stock solution so that the number
of moles of catalyst remained constant at 9.56 x 10-6 mol. A catalyst to substrate ratio of 1:100
was utilised whilst the substrate to oxidant ratio was varied in order to determine the optimum
ratio. The optimum reaction times were different for the two oxidant systems. The reaction
mixture was stirred in an oil bath maintained at 50 °C, after which an aliquot was removed,
treated with an excess amount of PPh3 and filtered through a silica plug prior to injecting 0.5 μl
into the GC for analysis and quantification of the products.
The yield was calculated based on the total moles of product formed divided by the initial moles
of substrate added into the reaction mixture and was expressed as a percentage, while the
percentage selectivity was expressed as moles of each product divided by the total moles of all
products.
3.
Results and discussion
3.1 Synthesis and characterization of complexes
This study is based on five new complexes bearing neutral SNS ligands bound in tridentate
pincer-type coordination modes to Cu(II) ions. A general synthetic route to the preparation of the
ligands and complexes is presented in Scheme 1, of which ligands L1a-c and their Ni(II)
complexes were recently reported32.
7

N
SH
R
N
Na/EtOH
CuCl2.2H2O
N
+
S
S
S
S
Cu
Cl
Cl
R
R
R
R
Cl
Cl
L1a-e
1a-e
Scheme 1: General synthetic route for the preparation of the SNS ligands L1 and CuSNS
complexes 1 where R = methyl (a), ethyl (b), butyl (c), cyclohexyl (d) and t-butyl (e).
Preparation of all the ligands (L1a-e) followed same procedure32 and synthesis of the complexes
involved addition of a solution of a ligand in methanol to a methanolic solution of CuCl2∙2H2O.
The observance of an instantaneous colour change served as the initial positive indicator of
successful metal coordination. All the complexes were thus prepared, isolated, purified and fully
characterised by IR, MS, elemental analysis, melting point determination and structural
determination by single crystal X-ray diffraction (SCXRD). An examination of the IR results
revealed some significant shifts in bands corresponding to the alkyl C−H stretch, bend and the
C−N stretching frequencies (Table 1).38 These shifts are indicative of successful coordination of
the SNS ligands (L1a-e) to Cu(II) centres yielding corresponding complexes (1a-e). The most
notable change in frequency is observed for the C−N stretch whereby shifts to lower
wavenumbers were seen for the complexes in comparison to the unbound ligands. This result is a
consequence of coordination of the ligand N-donor to the metal centre, which also correlates
with the SCXRD for L1e and 1e where a slightly longer C−N bond length was observed for the
complex (1.492 Å for L1e vs 1.496 Å for 1e).
Table 1: Selected IR data for ligands L1a-e and the corresponding complexes 1a-e.
Frequency (cm-1)
Compound
C−H stretch C−H bend
C−N stretch
1110
2843
2858
2846
2866
1437
1441
1453
1446
L1a
1a
1085
1109
L1b
1b
1087
8

2872
2861
2850
2850
2862
2862
1458
1450
1447
1447
1458
1461
1116
1088
1104
1069
1105
1071
L1c
1c
L1d
1d
L1e
1e
ESI-MS analysis of the ligands and complexes gave results that are in line with calculated
values. The ligand mass, together with a sodium adduct, is the most abundant ion, whereas for
the complexes, the predominant species correspond to a mass of the ligand and copper only, with
a loss of both chloride ligands, which dissociated during ionisation. Accurate combustion (CHN)
analysis data in addition to sharp melting points confirmed isolation of all the complexes in high
bulk purity. Due to the paramagnetic nature of complexes 1a-e, no resolvable NMR data were
possible, but detailed structural details were obtained from the SCXRD data presented below.
3.2 Structural analysis by single crystal X-ray diffraction
It is interesting to note that all of the SNS ligands presented in this study exist as oils at room
temperature, but an attempted complexation of ligand L1e to Ni(II) in EtOH via the commonly
utilized nickel precursor Ni(PPh3)2Br2 only yielded single crystals of a protonated bromide salt
of L1e. The crystals were grown by layering a DCM solution of what was initially believed to be
the SNS coordinated ‘Ni complex’ of L1e with Et2O. To our surprise, the crystal structure
presented in Fig. 1 does not contain any coordinated metal. Instead, the N-donor atom of the
SNS ligand was protonated via hydrogen transfer from the protic EtOH solvent used for the
reaction, which was most likely catalysed by the metal precursor Ni(PPh3)2Br2. The molecular
structure of the salt (Fig. 1) shows that the ethylene linkers [C5, C6] and [C8, C9], and the
central atoms defined by [C7, N1, H1 Br1] reside in two different planes that are nearly
perpendicularly at an angle of 88.5°. The [C7, N1, H1, Br1] plane can also be considered as a C2
rotation axis for the SNS skeletal backbone. Molecules of L1e exhibited non-classical hydrogen
bonding (N−H∙∙∙Br) separated by 2.191 Å, which is a relatively strong interaction in comparison
to literature reports.39,40 Hence, the crystal packing (II, Fig. 1), viewed along the b-axis, shows
9

intermolecular interactions in which two adjacent molecules are hydrogen bonded to two
corresponding bromido ions (N−H∙∙∙Br) in an almost perpendicular array.
Figure 1: Molecular structure (I) and crystal packing of the protonated salt of ligand L1e viewed
along 010 (II) with hydrogen atoms omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level.
For the metal complexes, single crystals of 1a were grown from a MeOH solution, while
concentrated solutions of 1b-1e in MeOH or DCM layered with Et2O or heptane, yielded dark
green crystals. The complexes crystallised with five coordinate ligand geometry around the
Cu(II) centres. The compounds are in the monoclinic (1a, 1b and 1e), triclinic (1c) or
orthorhombic (L1e and 1d) crystal systems (Table 2) with packing diagrams for all complexes
presented as supplementary materials (Fig S32 to S36).
10

Figure 2: Molecular structures of 1a (I), 1c (II) and 1e (III) with hydrogen atoms omitted for
clarity. Thermal ellipsoids are drawn at the 50% probability level.
Table 2: Selected crystallographic and structure refinement data for L1e and complexes 1a-1e.
L1e
1a
1b
1c
1d
1e
Empirical
formula
Formula
weight
Temperature
(K)
C13H30BrNS2
344.41
C7H17Cl2CuNS2
313.80
C9H21Cl2CuNS2 C13H29Cl2CuNS2 C17H33Cl2CuNS2
C13H29Cl2CuNS2
397.96
341.85
397.96
296(2)
450.04
100(2)
100(2)
173(2)
100(2)
296(2)
Crystal
Orthorhombic
Monoclinic
Monoclinic
Triclinic
Orthorhombic
Monoclinic
system
Space group
a (Å)
b (Å)
c (Å)
α (°)
P21212
10.1665(15)
27.674(4)
6.3937(9)
90
90
90
1798.9(4)
4
1.272
P21/n
P21/n
Pī
Pbca
P21/c
7.7103(4)
12.1139(7)
13.6506(8)
90
101.201(2)
90
1250.70(12)
4
1.6664
12.8374(4)
14.2641(5)
16.1896(3)
90
91.293(2)
90
2963.79(15)
4
1.532
8.58240(1)
14.5582(2)
15.4678(2)
105.5750(1)
98.6340(1)
96.7680(1)
1814.99(4)
4
12.1895(6)
21.8132(10)
31.6331(14)
90
90
90
8411.0(7)
16
1.4215
1.491
3806.8
15.0660(3)
9.7401(2)
13.5252(2)
90
110.5780(1)
90
1858.11(6)
4
1.4225
β (°)
γ (°)
Volume (Å3)
Z
ρcalc (g/cm3)
μ (mm-1)
F(000)
1.4563
1.716
839.7
2.502
728
2.466
647.6
2.088
1416.0
1.676
839.6
Crystal size
0.290 × 0.180 ×
0.140
4.268 to 56.578
0.641 × 0.34 ×
0.269
4.54 to 56.8
0.210 × 0.140 × 0.45 × 0.25 ×
0.090 0.25
7.616 to 55.758 4.6 to 57.22
0.844 × 0.656 ×
0.42
2.58 to 50.74
0.246 × 0.092 ×
0.078
5.08 to 56.76
(mm3)
2Θ range for
data
collection (°)
Index ranges
-11 ≤ h ≤ 13,
-18 ≤ k ≤ 36,
-8 ≤ l ≤ 5
8366
-10≤ h ≤10,
-16≤ k ≤15,
-17≤ l ≤18
28014
-16≤ h ≤16,
-18≤ k ≤18,
-21≤ l ≤21
164198
-11 ≤ h ≤ 11,
-19 ≤ k ≤ 19,
-20 ≤ l ≤ 20
34856
-14 ≤ h ≤ 14,
-26 ≤ k ≤ 26,
-37 ≤ l ≤ 38
170453
-20 ≤ h ≤ 19,
-12 ≤ k ≤ 13,
-17 ≤ l ≤ 17
29195
Reflections
collected
Independent
reflections
3932 [R(int) =
0.0240]
3113 [R(int) =
0.0237]
7054 [R(int) =
0.0604]
9196 [R(int) =
0.0181]
7720 [R(int) =
0.0808]
4604 [R(int) =
0.0271]
Data / restraints 3932 / 0 / 155
3113 / 0 / 121
7054 / 0 / 277
9199 / 0 / 369
7720 / 0 / 429
4604 / 0 / 189
11

parameters
Goodness-of- 1.049
1.016
1.108
1.055
1.070
1.051
fit on F2
Final R
R1 = 0.0273,
wR2 = 0.0532
R1 = 0.0180,
wR2 = 0.0493
R1 = 0.0289,
wR2 = 0.0575
R1 = 0.0267,
wR2 = 0.0684
R1 = 0.0415,
wR2 = 0.0879
R1 = 0.0242,
wR2 = 0.0573
indices [I
>=2σ (I)]
Final R
indices (all
data)
R1 = 0.0317,
wR2 = 0.0549
R1 = 0.0184,
wR2 = 0.0495
R1 = 0.0450,
wR2 = 0.0638
R1 = 0.0296,
wR2 = 0.0711
R1 = 0.0648,
wR2 = 0.1014
R1 = 0.0285,
wR2 = 0.0592
Largest diff.
peak and hole
0.43 and -0.69
0.37 and -0.61
0.54 and -0.38
1.96 and -1.07
0.82 and -0.62
0.61 and -0.28
(e.Å-3)
Table 3: Selected bond lengths (Å) and bond angles (°) for 1a, 1c and 1e.
Bond lengths (Å)
N1−Cu1
1a
1c
1e
2.0741(12)
2.5632(4)
2.3495(4)
2.2623(4)
2.2968(4)
2.0808(16)
2.5552(5)
2.3474(5)
2.2637(5)
2.2981(5)
2.0781(15)
2.5178(5)
2.4027(5)
2.2736(5)
2.2964(5)
S1−Cu1
S2−Cu1
Cl1−Cu1
Cl2−Cu1
Bond angles (°)
S1−Cu1−S2
N1−Cu1−S1
N1−Cu1−S2
N1−Cu1−Cl1
N1−Cu1−Cl2
109.594(14)
83.99(3)
112.938(17)
86.31(4)
123.036(17)
85.33(4)
86.40(4)
83.31(5)
83.73(4)
172.19(4)
93.52(3)
170.61(5)
92.41(5)
170.77(4)
92.54(4)
ORTEP diagrams showing the molecular structures of complexes 1a, 1c and 1e, presented in Fig.
2, show the SNS ligands coordinated in the expected tridentate fashion around each Cu(II)
centre. Previous results on a NiSNS complex based on ligand L1c we reported showed that the
complex displayed six coordinate, distorted octahedral geometry around the Ni(II) centre.32
Furthermore, analogous CuSNS complexes reported by Blomenkemper et al. and Hajifatheali et
al..24,25 showed five-coordinate distorted square pyramidal geometry of ligands. Hence, we
determined the tau parameter for complexes 1a, 1c and 1e to determine the actual geometry of
each Cu(II) coordination centre. It was found as τ = 1.04, 0.96 and 0.80 for 1a, 1c and 1e
respectively, indicative of distorted trigonal bipyramidal arrangement of ligands. In all three
complexes, the arrangements involve axially coordinated (N-donor and Cl1) and equatorially
12

coordinated Cl2 and two S-donors. For ease of comparison, structurally important bond lengths
and angles for 1a, 1c and 1e are presented in Table 3. The ‘bite angle’, defined by S1−Cu1−S2,
is acute for 1a (109.594(14)°), but much more open for 1c (112.938(17)°) and 1e (123.036(17)°)
implying a more accessible metal centre in the latter. This trend establishes a direct correlation
between steric size of the S-donor substituents and the bite angle, which may be related to the
ability of the central metal to coordinate and accommodate substrates during catalytic
transformations.19,24,41 The bond lengths corresponding to N1−Cu1, S1−Cu1 and S2−Cu1 are
comparable for 1a and 1c, however, for complex 1e the S1−Cu1 bond is slightly shorter at
2.5178(5) Å, whilst a longer S2−Cu1 bond is noted at 2.4027(5) Å. Overall, the S1−Cu1 bond
for all three complexes is significantly longer than reported literature values for related CuSNS
complexes, which are in the range of 2.3168-2.3625 Å.24
Selected bond lengths and angles for 1b and 1d are shown in Table 4. Complex 1b crystallises
with two independent molecules in its asymmetric unit cell, identified as fragments A and B
(Fig. 3). Although both fragments showed similar bond parameters, a slightly wider bite angle of
110.54(2)° was observed for fragment A in comparison to fragment B, with a S1−Cu1−S2 angle
of 107.66(2)°. The coordination geometry in complex 1b is similar to those of aforementioned
complexes, with trigonal bipyramidal configuration for both fragments (τ = 1.02 and 1.09 for A
and B respectively). Furthermore, the axial (N1, Cu1, Cl1) and the equatorial (S1, Cu1, S2, Cl2)
planes intersect at an average angle of 85.05° for the two fragments, which contributed to the
distorted geometries observed around the metal centres.
Table 4: Selected bond lengths (Å) and bond angles (°) for fragments A and B of complexes 1b
and 1d.
Bond lengths (Å)
1b
1d
A
B
A
B
N1−Cu1
2.0604(15)
2.6525(6)
2.3620(5)
2.2988(5)
2.2531(6)
2.0830(16)
2.6020(6)
2.3529(6)
2.2868(5)
2.2676(6)
2.076(3)
2.3688(10)
2.3733(10)
2.4699(10)
2.2493(10)
2.084(3)
2.5432(11)
2.3852(11)
2.2798(10)
2.2928(10)
S1−Cu1
S2−Cu1
Cl1−Cu1
Cl2−Cu1
Bond angles (°)
13

S1−Cu1−S2
N1−Cu1−S1
N1−Cu1−S2
N1−Cu1−Cl1
N1−Cu1−Cl2
110.542(19
82.44(4)
86.41(5)
171.89(5)
93.36(5)
107.656(19)
85.94(5)
161.36(4)
85.00(9)
84.47(9)
94.27(9)
152.38(9)
118.37(4)
83.66(10)
84.27(9)
171.02(9)
92.59(9)
82.76(5)
172.77(5)
93.98(5)
Figure 3: Molecular structure of 1b showing two independent molecules (A and B) in the
asymmetric unit cell with hydrogen atoms omitted and molecule B reoriented for clarity.
Thermal ellipsoids are drawn at the 50% probability level.
Complex 1d (Fig. 4) also crystallised with two crystallographically independent molecules (A
and B) in the asymmetric unit cell, but each fragment has a few unique features. For instance, the
geometry around the metal centre in fragment A is a distorted square pyramid (τ = 0.15), in
which the basal plane is occupied by N1A, S1A, S2A and Cl2A, whilst Cl1A occupied the apical
position. This is comparable in conformation to a similar CuSNS complex reported with bulkier
bromide ligands and a secondary amine backbone, as a consequence of which the reported bite
angle was more obtuse at 170.58(2)° vs 161.36(4)° for fragment A.42 In fragment B, a distorted
trigonal bipyramidal geometry was displayed (τ = 0.88), where N1B and Cl1B occupy the axial
positions forming an angle of 171.02(9)°, whilst S1B, S2B and Cl2B reside in the equatorial
plane. In comparison to 1a, 1b and 1c, a notably wider bite angle was observed for 1d and this
may be attributed to the bulkier cyclohexyl substituents in the chair conformation.
14

Figure 4: Molecular structure of 1d showing the two independent molecules (A and B) in the
asymmetric unit cell, with one conformation of the disordered cyclohexyl group displayed,
hydrogen atoms omitted and molecule B reoriented for clarity. Thermal ellipsoids are drawn at
the 50% probability level.
3.3 Oxidation of n-octane
All the prepared complexes were applied as catalysts in the oxidation of n-octane using either
TBHP or H2O2 as oxidant. All reactions were conducted in air under atmospheric pressure and a
reaction temperature of 50 °C. Various octane to oxidant ratios were tested in order to optimise
the quantity of oxidant required. Ratios of 1:12 and 1:9 for octane:TBHP and octane:H2O2
respectively, were determined as optimum for the best balance between product yield and
selectivity. Blank reactions in the absence of a catalyst produced yields of ca. 1% in 24 h for the
system with TBHP as the oxidant, whilst no conversion was observed in the absence of a catalyst
at the optimum reaction time for the H2O2 system (1 h). Conversions and product distribution of
the catalytic systems were benchmarked against a simple CuCl2 salt in order to determine the
influence of the SNS ligand system on Cu catalysed oxidation of n-octane.
15

For the 1/TBHP systems, both the conversion and product distribution were comparable to that
of the CuCl2 precursor (see Fig. S37 and S38). The results imply that in all cases, the addition of
TBHP led to ligand dissociation. Despite the absence of any ligand effect with TBHP as the
source of oxidant, a reasonable total yield of ~15% was recorded, which is higher than some of
our earlier results reported in the literature.43-45 A closer analysis of the distribution of products
indicates that the ketones represent the major products. Furthermore, it was found that the
reaction does not proceed in the presence of TEMPO, a common radical inhibitor, thus
suggesting a radical initiated pathway as the most plausible mode of action.
Table 5: Catalytic oxidation of n-octane at 50 °C with H2O2 as oxidant.a
Selectivity (%)
Catalyst
Yield to C-8 oxygenates (%)
-olb
-onec
C(1)d
27
34
23
36
26
1a
1b
1c
1d
1e
76
74
77
78
76
6
6
5
5
7
18
20
18
18
18
34
CuCl2
75
7
19
a All reactions were conducted for a time period of 1 h. Catalyst:octane:H2O2 = 1:100:900.
b 2-, 3- and 4-octanol. c 2-, 3- and 4-octanone. d 1-octanol and octanoic acid
The result obtained for the 1/H2O2 systems (Table 5) is much different to the reactions with
TBHP. An optimum octane to TBHP ratio of 1:9 was established with the highest yield to C-8
oxygenates obtained at 1 h reaction time. From the results, it is apparent that the complexes gave
relatively high combined yields (23-36%) of C-8 oxygenate products. The copper salt was also
quite active with a calculated yield of 34% to C-8 oxygenates. It is possible that a lag phase
initially occurs, where the rate of ligand dissociation varies for each of the catalysts, hence the
disparity in the yields obtained.
Analysis of product distribution profiles revealed that alcohol products predominate with the
1/H2O2 catalytic systems as opposed to the results of the 1/TBHP systems where ketones were
more abundant. This can be attributed to the much shorter reaction time of 1 h for the H2O2
16

reactions compared to the 24 h for TBHP whence the over oxidation of alcohols to ketones is
more likely. Furthermore, no distinction in the product selectivity was observed amongst the
catalysts in comparison to the uncomplexed CuCl2 salt. This suggests that the ligands have no
influence on the activity and selectivity of the catalysts, possibly due to their instability under the
oxidizing conditions. The X-ray crystal structural data of complex series 1 showed some degree
of asymmetry and/or disorder, brought about by the flexible backbone, which may be significant
contributing factors to the instability of the complexes in oxidizing environments.
In spite of the apparent lack of direct ligand based catalyst control, the significance of these
results is best appreciated when it is benchmarked against previous studies based on cobalt and
nickel complexes containing related SNS ligands.12,32 Yields of up to 10% and 23% were
obtained with the NiSNS and CoSNS catalysts respectively at 24 h using TBHP as the oxidant. It
is noteworthy that the copper system reported herein employ a much greener oxidant (H2O2)
working at a considerably reduced reaction time (1 h) and yielding much higher conversions of
n-octane (up to 36% yield to C-8 products). This clearly demonstrates that copper is a much
better choice of metal over either cobalt or nickel for the oxy-functionalisation of n-octane.
Moreover, the prevalent product group is the alcohols as opposed to the over oxidised ketones
observed with the TBHP (Ni or Co) systems.
From this study it is evident that the architecture of the ligand backbone, as well as the choice of
the metal that make up the catalyst, need to be carefully considered. Although the ligands were
flexible, they were rendered unstable under the conditions employed for the catalysis, yet copper
has shown significant enhancements over previously studied metals. Therefore, further
investigation is required for designing a flexible but stable ligand backbone that could potentially
improve the catalytic activity of the Cu(II) systems even further.
4.
Conclusion
In this work, pincer-type SNS ligands and their respective new copper complexes were
successfully synthesized using simple procedures. Characterisation by various techniques
confirmed the structural composition and bulk purity of all the prepared complexes. All the
complexes existed as mononuclear species characterised by five coordinate ligand geometry
17

around each Cu(II) centre. All the complexes were applied as catalysts for the oxidation of n-
octane using TBHP and H2O2 as oxidants. The Cu(II)/H2O2 catalyst systems produced yields of
23-36% with predominant C-8 alcohol selectivity. Hence, this study serves to extend our
understanding on the role of the SNS backbone on catalytic activity in n-octane oxidation. Lack
of steric rigidity of the methyl N-substituent rendered the resulting complexes unhindered,
flexible and unstable under the harsh oxidising environment of the catalytic process. Hence,
further exploration is still needed to achieve a balanced structure/activity relationship for
complexes bearing the modular SNS ligand backbone.
Acknowledgements
We are very grateful to the University of KwaZulu-Natal, c* change and the NRF for generous
financial support.
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20

Graphical Abstract
21

Highlights
A series of flexible SNS pincer ligands with a methyl functionality on the nitrogen backbone and
a variation of substituents on the S-donor atoms were prepared.
Coordination of the ligands to copper, using CuCl2∙2H2O as the metal precursor, yielded five-
coordinate complexes, which were fully characterised.
All the complexes were found to be catalytically active in the oxidation of n-octane using H2O2 as
the oxidant.
Relatively high yields to C-8 oxygenates were obtained at one hour with high selectivity to the
alcohol products.
22