Symmetric triazolylidene Ni(II) complexes applied as oxidation catalysts

Article abstract of DOI:10.1016/j.poly.2018.10.036

A set of related Ni(II) complexes of N-heterocyclic carbene ligands (NHC) [trans-X₂Ni(NHC)₂] (X = Cl, I) bearing linear straight chain alkyl wingtip substituents have been synthesised and fully characterised. Single crystal XRD data revealed symmetrically aligned Ni(II) centres within square planar coordination of trans halide, trans NHC ligands. The complexes were used for the catalytic oxidation of alkanes under mild conditions in conjunction with tert-butyl hydroperoxide as an oxidant. Under optimised reaction conditions, the catalytic results pointed to good activities of circa 15% and 19% for cyclohexane and n-octane respectively. Furthermore, the catalytic systems are shown to be very efficient for the oxidation of linear alcohols to corresponding ketones.

Full text of DOI:10.1016/j.poly.2018.10.036

Accepted Manuscript
Symmetric triazolylidene Ni(II) complexes applied as oxidation catalysts
Siyabonga G. Mncube, Muhammad D. Bala
PII:
S0277-5387(18)30672-7
https://doi.org/10.1016/j.poly.2018.10.036
POLY 13510
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Reference:
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13 October 2018
Please cite this article as: S.G. Mncube, M.D. Bala, Symmetric triazolylidene Ni(II) complexes applied as oxidation
catalysts, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.10.036
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Symmetric triazolylidene Ni(II) complexes applied as oxidation catalysts
Siyabonga G. Mncube and Muhammad D. Bala
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X54001, Durban
4000, South Africa
Abstract
A set of related Ni(II) complexes of N-heterocyclic carbene ligands (NHC) [trans-X2Ni(NHC)2]
(X= Cl, I) bearing linear straight chain alkyl wingtip substituents have been synthesized and fully
characterized. Single crystal XRD data revealed symmetrically aligned Ni(II) centres within
square planar coordination of trans halide, trans NHC ligands. The complexes were used for the
catalytic oxidation of alkanes under mild conditions in conjunction with tert-butyl hydroperoxide
as an oxidant. Under optimized reaction conditions, the catalytic results pointed to good activities
of circa 15 and 19% for cyclohexane and n-octane respectively. Furthermore, the catalytic systems
are shown to be very efficient for the oxidation of linear alcohols to corresponding ketones.
Keywords: NHC ligand; Ni complexes; Oxidation catalysis, n-Octane; Cyclohexane.
Introduction
The direct, catalytic oxidation of alkanes into value-added products under mild reaction conditions
is a topic of great interest to academia and industry. This is due to the immense potentials of the
functionalised products as building blocks for the pharmaceutical, petrochemical and fine
chemicals industries.[1-8] However, the direct, selective catalytic activation and oxidative
functionalisation of alkanes to alcohols has to date remained a challenge to researchers worldwide.
Recently, considerable effort is directed towards the catalytic oxidation of alkanes in homogeneous
solutions using earth-abundant first-row transition metals that include Ti[9], [10], V[11], Mn[5],
Fe[12-14], Co[15], Ni[16, 17] and Cu[7, 18]. These have become much more attractive in
comparison to late and noble transition metals which until recently have dominated this sphere.
* Correspondence to: e-mail- bala@ukzn.ac.za; fax- +27 31 260 3091
1

Increased interest in first-row transition metals in catalysis is due to obvious advantages, which
include high abundance, low price, low or no toxicity and unique or unusual catalytic
characteristics. This work is part of an ongoing study on the use of cheap, bio-compatible metals
such as Fe, Co and Ni for the catalytic oxidation of alkanes as a biomimetic strategy aligned to
simulate the high efficiencies of monooxygenase enzymes such as cytochrome P450 [12, 13, 15,
27, 28].
Since the first isolation of a stable, free N‐heterocyclic carbene (NHC) [19], metal complexes with
NHC ligands have increasingly found application in homogeneous catalysis.[20-23] The strong
σ‐donating effects of the NHC ligands toward metal centres make them good candidates for the
stabilization of metal ions during catalysis. In this regards, non-toxic and easy to prepare Ni-NHC
complexes are now widely used instead of the well-established but more expensive d8 homologues
Pt and Pd.[24-26]. In many instances, the steric and electronic properties of the NHC ligands exert
profound effects on the reaction outcome.[13, 27] Hence, herein we present one of the first use of
Ni-NHC pre-catalysts for the oxidative functionalisation of a variety of saturated substrates under
mild reaction conditions. We envisage that the introduction of hydrophobic alkyl chain groups as
wingtip N-substituents in the catalyst structure will positively influence its interaction with
hydrocarbon substrates for the preferential and selective functionalisation of the latter.
Results and discussion
All the triazolium iodide ligand precursor salts (Scheme 1) were synthesised via the standard
“click” 2+3 cycloaddition reactions of organic azides and phenylacetylene followed by subsequent
methylation with MeI. Their full characterisation and synthesis details are available in the
literature.[13, 29-30] At the beginning of this project, we adopted the method of Nolan and co-
workers for the synthesis of NHC-Ni(II) complexes from labile nickelocene (Cp2Ni).[31] But, we
soon realised that the direct reaction of triazolium salts with Cp2Ni at high temperatures (Route A,
Scheme 1) yielded a mixture of products, i.e. the monoNHC products (2), the intended products
(3) and recoverable quantities of unreacted Cp2Ni. Whilst such attempts proved to be successful,
however only low yields (ca. 10%) of some (3a-c) of the targeted products were isolated after
difficult workup procedures. Therefore, this route is synthetically inadequate for the preparation
of pre-catalysts required in relatively good quantities for full analysis and further studies. We then
2

explored the alternative metalation method (Scheme 1, Route B) that was recently reported by
Astakhov et al.,[32] whereby reaction of an in situ generated triazolylidene ligand with anhydrous
NiCl2-DME in dry acetonitrile resulted in the exclusive formation of the bis(NHC) complexes 4a-
e.
N
N
N
R
R
Ph
R
R
+
Ni
N
N
Cp2Ni
+
Route A
I
I
Ni
I
N
N
I
(i)
N
N
Ph
Ph
N
N
Ph
2
1
N
(ii)
Route B
3 (a-c)
R = C6H13 (3a)
R = C7H15 (3b)
R = C8H17 (3c)
N
N
N
Ph
R
Cl
Cl Ni
R
Ph
N
N
N
4 (a-e)
R = C3H7 (4a)
R = C4H9 (4b)
R = C5H11 (4c)
R = C6H13 (4d)
R = C8H17 (4e)
Scheme 1: Synthesis of Ni-NHC complexes. (i) Cp2Ni,THF, 80 °C (ii) NiCl2-DME, MeCN TEA,
80 °C.
All the complexes were fully characterized by 1H-, 13C-NMR, HRMS and EA. NMR spectral data
of the compounds synthesised via Route B indicate that the complexes were exclusively obtained
as the bisNHC versions with no contamination of any side products as observed via Route A
(Scheme 1). Isolated yields were much improved from very poor to moderate and high (50-72%).
In general, 1H-NMR resonances for the N-alkyl substituents for all the complexes were observed
as expected resonating as upfield multiplet signals (0.5 to 4 ppm), whilst resonances integrating to
10H between 6.35 and 8.92 ppm accounted for the aromatic C4-substituents of the ligands, thus
13
signifying a symmetric coordination of the NHC ligand platform. In the C-NMR spectra of all
the complexes (both series 3 and 4), resonances at δ 146-149 ppm represents the characteristic
metal-carbene (Ni-Ccarbene) bond which is a clear indication of NHC coordination to the metal
centre via the carbene C5 atom.
3

Further confirmation of the structural integrity of the complexes was obtained from X-ray
diffraction analysis of single crystals of compounds 3a-c (Figs. 1-3). Only single crystals suitable
for XRD of the diiodido versions of the complexes were isolated. All the structural data showed
that each complex contains a symmetric Ni(II) center within a square planar coordination of trans
halide, trans NHC ligands. This is consistent with the fact that 3a-c crystallised in the
centrosymmetric P-1 space group of the triclinic crystal system. The key structural highlight for
these compounds is the centre of inversion at each Ni that is located at the corners and face edges
of their unit cells. All the N-alkyl chains are linearly oriented away from each other to minimise
steric interactions. All the key crystallographic parameters are standard and no major deviations
were noted, for example, the Ni–I distances [2.50406(15) , 2.49400 and 2.4914(2) Å for 3a-c
respectively] and the Ccarbene-Ni(1)-Ccarbene angles [180.00 ] are consistent with those reported for
similar compounds.[32] For complex 3b, the heptyl N-substituent of the NHC ligand showed
strong anisotropic atomic displacements during refinement, suggesting a statistical positional
disorder that was taken into account for the final model (Fig. 2).
Fig. 1: Molecular structure of complex 3a with thermal ellipsoids drawn at the 50% probability
level. Selected distances (Å) and angles (°): Ni(1)–C(8) 1.909(2), Ni(1)–I(1) 2.504(6), C(8)-Ni1-
C(8) 180.00, I(1)-Ni(1)-I(1) 180.00, C(8)–Ni(1)–I(1) 88.65(5).
4

Fig. 2: Molecular structure of complex 3b with thermal ellipsoids drawn at the 50% probability
level. Selected distances (Å) and angles (°): Ni(1)–C(8) 1.907(2), Ni(1)–I(1) 2.494 (7), C(8)-Ni1-
C(8) 180.00, I(1)-Ni1-I(1) 180.00, C(8)–Ni(1)–I(1) 88.76(7).
Fig. 3: Molecular structure of complex 3c with thermal ellipsoids drawn at the 50% probability
level. Selected distances (Å) and angles (°): Ni(1)–C(8) 1.907(3), Ni(1)–I(1) 2.491(2), C(8)-Ni1-
C(8) 180.00, I(1)-Ni1-I(1) 180.00, C(8)–Ni(1)–I(1) 88.65(5).
5

Electrochemistry
The continued success of NHCs as ancillary ligands in organometallic chemistry and
homogeneous catalysis is largely due to the seemingly endless possibility for researchers to fine-
tune the steric and electronic properties of complexes via azolium ring and wingtip variations,
which routinely alter the topography around metal centers. In many regards, variations in
electronic properties of organometallic catalysts usually have the greater influence on their
catalytic activity, which could be probed using cyclic voltammograms (CV). Hence, we have
determined the CV of compounds 4a-e as a very convenient technique to observe the impact of
changes in electronic properties around the Ni(II) metal centres with subtle changes in ligand
architecture. The result presented in Fig. 4 and summarised in Table 1 shows the relative
electrochemical responses recorded in DCM/MeCN.
Fig. 4: Comparison of the electrochemical response of 4a-e probed by CV.
Table 1: Electrochemical data[a] of Ni-NHC complexes in DCM/MeCN solution at 25 °C.
Complex
E1/2 (CV) [V]
0.829
0.831
0.832
0.833
4a
4b
4c
4d
4e
0.835
[a] Potential measured vs Ag/AgCl (0.004m, 0.1 M NBu4BF4) in 2 mM DCM/MeCN solutions at
a scan rate of 100 mV s-1.
6

All the compounds showed reversible oxidation responses circa 0.832 V which correlates well with
findings in the literature which according to Crabtree et al. corresponds to the NiII/III oxidation
wave.[33-34] The influence of the wingtip alkyl N-substituents (-R, Scheme 1) is conveniently
evaluated by comparing half wave potentials (E1/2) of 4a-e. Noteworthy, is the fact that increase
in the length of the alkyl substituents had a negligible influence on the values of the oxidation
potentials. This perhaps indicates that any differences in catalytic activities of the complexes will
largely be due to structural and geometric changes rather than any major deviations in electronic
properties.
Oxidation of n-octane
The oxidative functionalisation of n-octane is arguably the most attempted amongst straight chain
alkanes due to the fact that is it a mid-range substrate with properties intermediate between those
of light alkanes (gases that are difficult to handle and requiring specialised equipment for handling)
and heavy alkanes (waxes that are difficult to oxidise under mild conditions). Hence, activities of
the two sets of complexes were explored in the oxidation of n-octane with the results presented in
Table 2.
Table 2: Ni(II) complexes promoted catalytic oxidation of n-octane.a
Catalyst Conversion
(%)
Product distribution (%)b
octanones
(4-;3-;2-)
71
octanols
(4-;3-;2-;1-)
27
octanal
octanoic
acid
0
NiCl2
3a
6
8
2
7
(12;31;28)
43
(8;12;7;0)
46
4
0
(5;15;23)
(4;15;20;7)
3b
9
41
53
6
(6;14;21)
(7;17;21;8)
3c
4a
11
16
39
57
4
4
0
6
(7:14:18)
59
(5:19:23:10)
31
(8:18:33)
(4;10;15;2)
7

4b
4c
4d
4e
16
17
19
19
57
(8;17;32)
52
34
(4;11;15;4)
40
4
4
5
5
5
4
2
3
(6;18;28)
48
(3;11;21;5)
45
(7;16;25)
44
(5;12;21;7)
48
(4;16;24)
(6;15;22;5)
aConditions for all reactions: solvent (MeCN) = total vol. 5 mL; n-octane = 1 M; oxidant (tBuOOH) =14 M equivalent;
catalyst = 1 mM ; 80 °C; 12 h. b Total conversion based on product formed = [products]/[initial conc. of substrate]o x
100. Selectivity (%) towards ketone formation = molar amount of ketone/total molar amount of products (alcohol and
ketone) x 100. Conversion data are averaged over two runs that agree within ±5%.
Optimisation and control studies at lower temperatures showed low substrate conversions and in
the absence of a catalyst and/or oxidant, the results showed 0% conversion of the substrate. Control
reactions utilising the NiCl2 precursor salt as catalyst resulted in circa 6% conversion of n-octane.
Screening of the diiodido complexes (3a-c) under optimised conditions showed low catalytic
activities (8, 9 and 11% respectively) that are generally less than the dichlorido analogues 4a-e.
This is in agreement with the ease of dissociation of the iodide ligand as compared to lighter
halogens in the presence of coordinating solvents such as acetonitrile to form cationic complexes
that interact poorly with alkanes. Hence, the oxidation of n-octane with catalysts 4a-e produced
up to 19% substrate conversion to a mixture of isomeric alcohols and ketones. Also, compared to
the NiCl2 control reaction, the improved substrate conversion is indicative of the influence of
coordinated NHC ligands on the Ni(II) catalysts. The influence of the wingtip N-substituents on
the catalytic results is minimal but noticeable with total conversion showing gradual increase (16%
for 4a vs 19% for 4d-e) as the length of the substituent increased, which tallies with the CV data
discussed earlier. In addition, the pattern of product distribution follows this trend with an increase
in alcohol production with increase in N-alkyl chain length. Furthermore, in all sets of catalysts,
C(2) products (mainly 2-one) dominated while the terminal C(1) position is the least reactive and
the catalysts are more reactive to internal carbons in general.
Oxidation of cyclohexane
8

In the application of complexes 4a-e as catalysts for oxidation catalysis, the oxidation of
cyclohexane was tested due to the importance of the product [ketone-alcohol (K-A oil)] as a
feedstock to the polymer and allied industry.[35-36] Cyclohexane to oxidant ratio of 1:14 and 12h
reaction time was adopted for all the studies reported in Table 3. As expected, K-A oil was the
main product with varying compositions of the ketone and alcohol in the mixture depending on
the catalyst used.
Table 3: Peroxide promoted catalytic oxidation of cyclohexane based on catalysts 4a-e.a
OH
O
4a-e
CH3CN, 70 oC,
t-BuOOH
+
Entry Catalyst Conversion
Ketone (K)
Alcohol (A)
K/A
(%)
15
15
14
14
12
(%)
78
69
71
61
59
(%)
22
31
29
39
41
1
2
3
4
5
4a
4b
4c
4d
4e
3.5
2.2
2.4
1.6
1.4
aConditions for all reactions: Conditions for all catalytic reactions: same as Table 1. Total conversion based on product
formed = [products]/[initial conc. of substrate] x 100. Selectivity (%) towards ketone (or alcohol) formation = molar
amount of ketone (or alcohol)/total molar amount of products (alcohol and ketone) x 100.
The results suggest that the catalysis is insensitive to variations in the catalyst structure with total
maximum conversion of 15%. This is not entirely surprising considering that the substrate is
linearly planar and was unaffected by the subtle variations in the NHC ligands’ architecture. Hence,
the total conversion of the substrate and the relative ketone/alcohol (K/A) ratio remained
essentially the same. In addition, cyclohexanone is the more dominant product in all the systems
tested with K/A values decreasing as the wingtip alkyl chain size increased. However, in
comparison, catalysts 4a-e are slightly more active on n-octane than cyclohexane.
Oxidation of alcohols
Previous reports [4-7, 13, 18] have affirmed that alcohols are the primary products of alkane
oxidation. It was proposed that overoxidation of the primary products led to the domination of
ketones as the final products often observed in many homogenous alkane oxidation reactions. In
an effort to shed more light on this phenomenon, we tested a number of alcohols (1-, 2-, 3- and 4-
9

octanol) towards further oxidation under the same conditions utilised for the n-octane oxidation
reactions (vide supra). These new substrates are the primary and only products expected in the
direct oxidation of n-octane if there is no overoxidation with complex 4a (the most effective in the
production of ketones, Tables 2 & 3). The results presented in Fig. 5 confirm that accumulation of
ketones is gradual and time dependent; with a maxima at circa 12 h reaction time. In general, the
2- and 3- octanones accumulated more readily with conversions of 72 and 63% respectively at 12
h. This is in agreement with the product profile in Table 1 for the direct oxidation of n-octane and
also supports many reports that indicate the C(2) position as the most reactive site in n-octane
oxidation.[15, 37-41]
aldy+acid
2-one
80
70
60
50
40
30
20
10
0
3-one
4-one
0
4
8
12
Time (h)
16
20
24
Fig. 5: Peroxide promoted catalytic oxidation of alcohols (1-, 2-, 3- and 4- octanol) utilising complex 4a as catalyst.
Conditions for all catalytic reactions: same as Table 1.
Mechanistic considerations
The addition of a stoichiometric amount of a radical scavenger (diphenylamine, PH2NH) to the
most active alkane oxidation mixture retarded (6% yield) the reaction. [42-43] This is not
surprising as it conforms to well-established reports in the literature [44-47] that identified the
formation of alkyl(hydro)peroxo radicals as the main pathway to the formation of mainly alcohol
and ketone products in the oxidation of paraffin substrates with peroxide oxidants. In the systems
reported herein, we propose that the oxidation process proceeded via a Fenton-like mechanism
involving reactive Ni(II) species stabilised by the NHC ligands. In particular, Hikichi and
10

coworkers [47] have reported on a Ni(II) alkylperoxo complex [NiII(OOtBu)TpiPr2] generated in
the presence of tBuOOH for the oxidation of cyclohexane with cyclohexanone as the major
product. On the basis of which they proposed involvement of the Ni(II) alkylperoxo radical in the
reaction.
Perspectives on nickel based alkane oxidation catalysis
There are very few reported examples of Ni-catalysed oxidation of especially straight chain alkanes
under mild reaction conditions. Catalytic oxidation of cyclohexane is the most reported subject in
this field; hence, Table 4 summarises comparable catalytic systems based on cyclohexane. Due to
variability in the reporting of productivity data, some of the results presented in Table 4 were
normalised to the catalyst concentration and expressed in units of turnover numbers (TON) or
conversion (%) for ease of comparison.
Table 4: A comparison of the catalytic activities of Ni catalysts for the oxidation of cyclohexane.
Entry Catalyst[a]
Reaction conditions
Solvent/Temp/Oxidant
Conv.[b] TON[c]
1
Complex 4a
ACN/ 80°C/ tBuOOH
15
150
149
656
234
622
533
2[d]
3[e]
4[f]
5[g]
6[h]
[Ni(TPA)(CH3CN)2](BPh4)2 DCM:ACN/ RT/ m-CPBA
14.9
[Ni2(TPA)2(µ-NO3)2](BPh4)2 DCM:ACN/ RT/ m-CPBA 8.6
Ni(Tp)
DCM/ 40°C/ m-CPBA
0.72
[Ni(L)(CH3CN)2](BPh4)2
[Ni(L)(CH3CN)](BPh4)2
DCM:ACN/ RT/ m-CPBA 8.8
DCM:ACN/ RT/ m-CPBA 7.6
[a] The most active catalyst from each publication is quoted.
[b] Conv. (total conversion) = moles of products/ moles of cyclohexane x100.
[c] TON = moles of products/ moles of catalyst.
[d] TPA = tris(2-pyridylmethyl)amine.[48]
[e]
TPA = tris(2-pyridylmethyl)amine. NO3 = nitrate, co-ligand.[49]
[f]
Tp = hydrotris(pyrazolyl)borate.[51]
[g] L = N,N-diethyl-N’-N’-bis(pyrid-2-ylmethyl)ethane-1,2-diamine.[52]
[h] L = N-benzyl-N’N’,N’-tris(pyrid-2-yl-methyl)-ethylenediamine.[53]
11

In this work, the most active catalyst proceeded very effectively with total conversion of circa 15%
with high selectivity to ketone products (78%). A close result was reported by Nagataki et al. [48]
reported with m-chloroperbenzoic acid (m-CPBA) promoted by a simple NiII(TPA) complex at
room temperature (entry 2). The catalytic reaction proceeded very effectively with total conversion
of circa 15% giving higher selectivity to alcohol products (89%) possibly suggesting a different
−
reaction pathway. The same group also reported [49] that introducing NO3 as a co-ligand (entry
3) decreased the oxidation reaction (giving total conversion of 8.6%). Similar phenomenon was
observed by Hitomi et al.[50] using FeII(TPA) as a catalyst. Hikichi et al. [51] have reported
catalytic cyclohexane oxidation with m-CPBA catalyzed by Ni(II) complexes with
hydrotris(pyrazolyl)borate ligands. The catalytic reactions proceeded ineffectively with total
conversion of 0.72% mainly aimed at achieving high alcohol selectivity (A/K =32) and TON (234).
In a similar vein, Palaniandavar and coworkers [52] used m-CPBA catalyzed by Ni(II) complexes
with tripodal N4 ligands for which they reported circa 9% conversion of cyclohexane with TON
of 622 and an impressive 90% cyclohexanol selectivity. The same group reported even higher
selectivity to cyclohexanol (92%) with pentadentate N5 ligands, although the conversion and TON
values are lower for these systems (entry 6). [53]
Conclusion
In the present study, highly symmetric square planar Ni(II) NHC complexes were successfully
synthesised and fully characterised. Application of the metal complexes as catalysts in alkane
oxidation resulted in a mixture of isomeric oxidised products, mainly ketones and alcohols. Based
on the results, we conclude that the trends in catalytic activities of the metal complexes are
attributable to subtle changes around each Ni(II) centre as a result of NHC ligand structural
variations. Thus, for the oxidation of cyclohexane, the observed trend is due to a decline in
substrate accessibility to the Ni centres as the size of the geometrically unconstrained alkyl
substituents increased from 4a to 4e. Whilst for the oxidation of the straight chain substrate n-
octane, it is most likely due to increased lipophilicity of 4a-e as the size of the alkyl substituents
increased, thereby improving their compatibility with the non-polar substrate. Finally, 4a was
found to be an effective catalyst for the oxidation of alcohols to ketone products, which serves to
shed more light on the sequence of the catalytic oxidation of alkanes to the major (ketone and
alcohol) products.
12

Experimental
All solvents were freshly distilled before used. All triazolium salts 1 were prepared as described
in published literature. [13, 22-24] All NMR experiments were done using a 400 MHz Bruker
ultrashield spectrometer with samples dissolved in deuterated solvents. Chemical shifts (δ) in ppm
are reported with respect to tetramethylsilane (TMS) as internal standard.
General procedure for the synthesis of the diiodido Ni(II) complexes (3a-c)
The reported method [31] was modified: To a solution of triazolium salts 1 in THF (15 mL)
nickelocene (0.5 equiv) was added. The mixture was stirred at 90 °C for 24 h. All volatiles were
removed under vacuum and the residue was extracted with hot toluene (20 mL). The extract was
then filtered through a plug of Celite. Combined extracts were concentrated under vacuum and
passed through a chromatography column that was eluted with EtOAc/pentane (1:1). Coloured
fractions were collected and the second fraction was evaporated to dryness to give analytically
pure 3(a-c).
Characterisation data:
(3a). Yield: 0.081 g (10 %) of brown-red crystals. 1H NMR (CDCl3, 400 MHz): δ 0.95 (t, JHH
=
7.5 Hz, 6H, 2CH3), 1.40 (m, 4H, 2CH2), 1.91-2.00 (m, 8H, 4CH2), 2.61 (m, 4H, 2CH2), 4.26 (s,
13
6H, 2CH3), 4.54 (t, JHH = 7.6 Hz, 4H, 2CH2), 7.39-7.79 (m, 10H, 2Ar). C{1H} NMR (CDCl3,
100.6 MHz): δ 15.61, 25.92, 29.60, 35.52, 41.76, 43.43, 54.23, 119.54, 125.69, 126.94, 127.94,
128.06, 128.16, 128.54, 128.69, 128.80, 128.81, 128.98, 129.53, 129.53, 131.88, 148.14. . Anal.
Calcd for C30H42I2N6Ni (799.19): C, 45.09; H, 5.30; N, 10.52. Found: C, 45.11; H, 5.41; N, 10.53.
(3b) Yield: 0.081 g (10 %) of red crystals. 1H NMR (CDCl3, 400 MHz): δ 0.77 (t, JHH =7.5 Hz,
6H, 2CH3), 0.11-1.39 (m, 8H, 4CH2), 2.01 (m, 4H, 2CH2), 3.79 (s, 6H, 2CH3), 4.26 (t, JHH = 7.6
13
Hz, 4H, 2CH2), 7.48-7.68 (m, 10H, 2Ar). C{1H} NMR (CDCl3, 100.6 MHz): δ 14.08, 22.62,
26.20, 28.89, 29.14, 29.26, 29.44, 31.77, 39.32, 54.63, 121.78, 129.71, 131.98, 142.95, 149.09.
Anal. Calcd for C32H46I2N6Ni (826.12): C, 46.46; H, 5.60; N, 10.16. Found: C, 46.47; H, 5.66; N,
10.16.
(3c). Yield: 0.065 g (9 %) of red crystals. 1H NMR (CDCl3, 400 MHz): δ 0.97-1.03 (t, JHH =7.4
Hz, 6H, 2CH3), 1.40-1.56 (m, 10H, 5CH2), 193 (m, 4H, 2CH2), 4.07 (s, 6H, 2CH3), 4.36 (t, JHH
=
13

7.6 Hz, 4H, 2CH2), 7.42-7.83 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 11.09, 11.18,
22.91, 23.76, 39.84, 52.03, 56.62, 119.81, 121.44, 152.27, 125.66, 158.52, 125.84, 128.89, 129.09,
129.13, 129.92, 13061, 143.57, 147.57. Anal. Calcd for C34H50I2N6Ni (855.30): C, 47.75; H, 5.89;
N, 9.83. Found: C, 47.75; H, 5.90; N, 9.85.
General procedure for the synthesis of the dichlorido Ni(II) complexes (4a-e)
The reported method [32] was modified: To a solution of triazolium salts 1 in THF (15 mL) was
added anhydrous NiCl2 (0.5 eq), and Et3N (4 eq) in dry acetonitrile (20 mL) and heated under
reflux for 6 h. All volatiles were removed under reduced pressure, and the residue obtained was
chromatographed on a silica gel column, with neat CHCl3 as eluent to give 4.
Characterisation data:
(4a). Yield: 0.451 g (56 %) of brown crystals. 1H NMR (CDCl3, 400 MHz): δ 0.94 (t, JHH = 7.4
Hz, 6H, 2CH3), 2.01-2.03 (m, 4H, 2CH2), 4.13 (s, 6H, 2CH3), 4.38 (t, JHH = 7.6 Hz, 4H, 2CH2),
7.46-7.56 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 1.67, 14.86, 25.39, 43.85, 5.95,
121.97, 131.27, 132.75, 142.60, 148.41. Anal. calculated for C24H30Cl2N6Ni (532.13): C, 54.17;
H, 5.68; N, 15.81; found: C, 54.18; H, 5.67; N, 15.81. TOF MS+ (ESI) m/z calculated for
C24H30N6Ni+ [M+Na]+ : 484.218; found 484.299.
(4b). Yield: 0.532 g (68 %) of yellow-brown powder. 1H NMR (CDCl3, 400 MHz): δ 0.01-0.06
(t, JHH = 7.4 Hz, 6H, 2CH3), 1.49-1.55 (m, 4H, 2CH2), 2.09-2.12 (m, 4H, 2CH2), 4.33 (s, 6H,
2CH3), 4.70-4.76 (t, JHH = 7.6 Hz, 4H, 2CH2), 7.66-7.80 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3,
100.6 MHz): δ 14.05, 28.98, 29.04, 31.70, 50.45, 119.35, 125.69, 128.07, 128.28, 128.48, 128.82,
130.64, 130.75, 147.3. Anal. calculated for C26H34Cl2N6Ni (560.18): C, 55.75; H, 6.12; N, 15.00;
found: C, 55.81; H, 6.14; N, 15.04. TOF MS+ (ESI) m/z calculated for C26H34N6Ni+: 488.220;
found 488.227.
(4c). Yield: 0.602 g (72 %) of brown powder. 1H NMR (CDCl3, 400 MHz): δ 0.82-0.83 (t, JHH
=
7.4 Hz, 6H, 2CH3), 1.26-1.36 (m, 12H, 6CH2), 2.03 (m, 4H, 2CH2), 4.25 (s, 6H, 2CH3), 4.74 (t,
JHH = 7.6 Hz, 4H, 2CH2), 7.52-7.69 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 14.08,
22.62, 26.66, 28.81, 29.10, 31.77, 39.12, 44.64, 127.52, 127.71, 128.01, 128.85, 129.06, 148.54.
Anal. calculated for C28H38Cl2N6Ni (588.24): C, 57.17; H, 6.51; N, 14.29; found: C, 57.18; H,
14

6.52; N, 14.30. TOF MS+ (ESI) m/z calculated for C28H38N6ClNi+ [M-Cl]+ : 551.220; found
551.221.
(4d). Yield: 0.582 g (61 %) of brown powder. 1H NMR (CDCl3, 400 MHz): δ 0.95-0.97 (t, JHH
=7.5 Hz, 6H, 2CH3), 1.42 (m, 12H, 6CH2), 2.14 (m, 4H, 2CH2), 4.35 (s, 6H, 2CH3), 4.70 (t, JHH
=
7.6 Hz, 4H, 2CH2), 7.66-7.84 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3, 100.6 MHz): δ 22.62, 26.66,
28.81, 29.10, 31.77, 39.12, 44.64, 127.52, 127.71, 128.01, 128.85, 129.06, 148.54. Anal. calculated
for C30H40Cl2N6Ni (588.24): C, 58.47; H, 6.87; N, 13.64; found: C, 58.50; H, 6.88; N, 13.671.
TOF MS+ (ESI) m/z calculated for C30H42N6Ni+ [M+Li]+ : 551.298; found 551.308.
1
(4e). Yield: 0.611 g (65 %) of brown powder. H NMR (CDCl3, 400 MHz): δ 0.78-0.81 (t, JHH
=7.4 Hz, 6H, 2CH3), 1.19-1.76 (m, 10H, 5CH2), 1.78-1.82 (m, 4H, 2CH2), 3.93-3.95 (s, 6H, 2CH3),
4.01 (t, JHH = 7.6 Hz, 4H, 2CH2), 7.34-7.56 (m, 10H, 2Ar). 13C{1H} NMR (CDCl3, 100.6 MHz):
δ 1.87, 13.92, 22.32, 22.44, 25.83, 29.14, 30.96, 38.61, 54.37, 65.83, 127.67, 128.69, 128.84,
129.51, 129.68, 131.81, 143.25, 148.43. Anal. calculated for C34H50Cl2N6Ni (670.28): C, 60.73;
H, 7.50; N, 12.50; found: C, 60.77; H, 7.53; N, 12.52. TOF MS+ (ESI) m/z calculated for
C34H50N6Ni [M+Li]+ : 607.36; found 607.30.
Typical procedure for the oxidation of substrates
Oxidation reaction was carried by mixing 1.0 mL of a 10.0 mM stock solution of catalyst in MeCN,
1.0 mL of a 1.0 M stock solution of the substrate (n-octane, cyclohexane or alcohol) and tert-butyl
hydroperoxide (1.0 mL of 70% aq., 14 M). The reaction volume was kept constant (5 mL) by
adding MeCN. The reaction mixture was stirred for the indicated time and temperature.
All products analyses by GC followed the method developed by Shul’pin [45, 46], using a Fisons
Instruments model 8160 gas chromatograph equipped with a DBWAX capillary column (column
length: 30 m; internal diameter: 0.32 mm; film: 0.25 mm) with helium as carrier gas and FID
detector. Blank experiments confirmed that in the absence of a catalyst, no products (negligible)
of alkane or toluene oxidation were observed.
X-ray structure determination
15

The structures were solved by direct methods using SHELXS.[54] Non-hydrogen atoms were first
refined isotropically followed by anisotropic refinement by full matrix least-squares calculations
based on F2 using SHELXS. Hydrogen atoms were first located in the difference map then
positioned geometrically and allowed to ride on their respective parent atoms. Diagrams were
generated using SHELXTL, PLATON[55] and ORTEP-3[56].
Electrochemistry
Cyclic voltammetry was performed using a standard three-electrode configuration: a platinum disk
electrode was used as the working electrode, a platinum wire as the auxiliary electrode, and a silver
electrode as reference, separated from the test solution by a fine porosity frit. The electrochemical
measurement of respective Ni-NHC complexes (2 mM) in DCM/MeCN and 0.1 M NBu4BF4 was
used as the supporting electrolyte. Data workup was performed on OriginPro v8.0988.
Appendix A. Supplementary data
CCDC 1566136, 1571157 and 1566137 contains the supplementary crystallographic data for
3a, 3b and 3c respectively. These data can be obtained free of charge via
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; or e-mail:
deposit@ccdc.cam.ac.uk.)
Acknowledgements
We are very grateful to the NRF, University of KwaZulu-Natal and Centre of Excellence in
Catalysis (c*change) for generous financial support. We thank Mr Sizwe Zamisa for X-ray
crystallographic data collection and refinement.
References
1.
Mac Leod T.C.O., Kopylovich M.N., Guedes da Silva M.F.C., Mahmudov K.T. and
Pombeiro A.J.L., Applied Catal. A: General, 2012, 439-440.
2.
3.
Labinger J.A. and Bercaw J.E., Nature, 2002, 417, 507-514.
Peris E. and Crabtree R.H., C. R. Chim. , 2003, 6, 33-37.
16

4.
5.
6.
7.
Bonon A.J., Mandelli D., Kholdeeva O.A., Barmatova M.V., Kozlov Y.N. and Shul’pin
G.B., Appl. Catal. A, 2009, 365, 96-104.
Shul’pin G.B., Kozlov Y.N., Nizova G.V., Süss-Fink G., Stanislas S., Kitaygorodskiy A.
and Kulikova V.S., J. Chem. Soc. Perkin Trans. , 2001, 2, 1351-1371.
Kirillova M.V., Kirillov A.M., Mandelli D., Carvalho W.A., Pombeiro A.J.L. and Shul’pin
G.B., J. Catal., 2010, 272, 9-17.
Kirillova M.V., Kozlov Y.N., Shul’pina L.S., Lyakin O.Y., Kirillov A.M., Talsi E.P.,
Pombeiro A.J.L. and Shul’pin G.B., J. Catal., 2009, 268, 26-38.
Sen A., Acc. Chem. Res., 1998, 31, 550-557.
8.
9.
10.
11.
Poladi R.H.P.R. and Landry C.C., Microporous and Mesoporous Mater. 2002, 52, 11-18.
Su B., Cao Z.C. and Shi Z.J., Acc. Chem. Res., 2015 48, 886-896.
Shul’pina L.S., Kirillova M.V., Pombeiro A.J.L. and Shul’pin G. B., Tetrahedron, 2009,
65, 2424-2429.
12.
13.
14.
15.
16.
Kadwa E., Bala M.D. and Friedrich H.B., Appl. Clay Sci., 2014, 95, 340–347.
Mncube S.G. and Bala M.D., J. Mol. Liq., 2016, 215, 396-401.
Yiu S.M., Man W.L. and Lau T.C., J. Am. Chem. Soc. , 130, 10821-10827.
Soobramoney L., Bala M.D. and Friedrich H.B., Dalton Trans., 2014, 43, 15968-15978.
Tordin E., List M., Monkowius U., Schindler S. and Knör G., Inorg. Chim. Acta, 2013,
402, 90-96.
17.
18.
Nagataki T., Tachi Y. and Itoh S., Chem. Commun., 2006, 22, 4016-4018.
Mac Leod T.C.O., Kirillova M.V., Pombeiro A.J.L., Schiavon M.A. and Assis M.D.,
Applied Catal.s A: General 2010, 372, 191-198.
19.
20.
21.
22.
23.
Arduengo III A. J., Harlow R. L., and Kline M. J. Am. Chem. Soc., 1991, 113, 361-363.
Wanzlick H. W. and Schonherr H. J., Angew. Chem. Int. Edn Engl., 1968, 7, 141-142.
Ofele K., J. Organomet. Chem., 1968, 12, 42-43.
Dıéz-González S. and Nolan S. P., Coord. Chem. Rev., 2007, 251, 874-883.
Jacobsen H., Correa A., Poater A., Costabile C. and Cavallo L., Coord. Chem. Rev., 2009,
253, 687-703.
24.
Schweinfurth D., Pattacini R., Strobel S. and Sarkar B., Dalton Trans., 2009, 42, 9291-
9297.
25.
26.
Schweinfurth D., Strobel S. and Sarkar B., Inorg. Chim. Acta. , 2011, 374, 253-260.
Kilpin K.J., Gavey E.L., McAdam C.J., Anderson C.B., Lind S.J., Keep C.C., Gordon K.C.
and Crowley J.D., Inorg. Chem., 2011, 50, 6334-6346.
27.
28.
29
30.
31.
Ibrahim H. and Bala M.D., New J. Chem. , 2016, 40, 6986-6997.
Cele M.N., Friedrich H.B., Bala M. D., Catal. Commun., 57 (2014) 99-102.
Yacob Z., Shah J., Leistner J. and Liebscher J., Synlett, 2008, 15, 2342-2344.
Li P. and Wang L., Lett. Org. Chem., 2007, 4, 23-26.
Kelly R. A., Scott N.M., Díez-González S., Stevens E. D. and Nolan S. P., Organometallics
2005, 24, 3442-3447.
32.
33.
34.
35.
Astakhov A.V., Khazipov O.V., Degtyareva E.S., Khrustalev V.N., Chernyshev V.M. and
Ananikov V.P., Organometallics, 2015, 34, 5759-5766.
Luca O.R., Thompson B.A., Takase M.K. and Crabtree R.H., J. Organomet. Chem., 2013,
730, 79-83.
Brewster T.P., Blakemore J.D., Schley N.D., Incarvito C.D., Hazari N., Brudvig G.W. and
Crabtree R.H., Organometallics, 2011, 30, 965-973.
Schuchardt U., Carvalho W.A. and Spinace´ E.V., Synlett, 1993, 10, 713-718.
17

36.
37.
Schuchardt U., Cardoso D., Sercheli R., Pereira R., da Cruz R.S., Guerreiro M.C., Mandelli
D., Spinace E.V. and Pires E. L., Appl. Catal. A, 2001, 211, 1-17.
Kirillov A.M., Kirillova M.V., Shul'pina L.S., Fig.iel P.J., Gruenwald K. R., Guedes da
Silva M.F.C., Haukka M., Pombeiro A.J.L. and Shul'pin G.B., J. Mol. Catal. A: Chem.,
2011, 250, 26-34.
38.
39.
Smith L.J.R., Iamamoto Y. and Vinhado F.S., J. Mol. Catal. A: Chem., 2006, 252, 23-30.
Bailey A.J., Griffith W.P. and Savage P.D., J. Chem. Soc., Dalton Trans., 1995, 3537-
3542.
40.
41.
42.
Naicker D., Friedrich H.B. and Omondi B., RSC Adv., 2015, 5, 63123-63129.
Ikhile, M.I.; Bala, M.D., Acta Crystallographica Section E, 2010, E66, o3121.
Kirillov A.M., Kopylovich M.N., Kirillova M.V., Karabach E. Yu., Haukk M Guedes da
Silva M.F.C., Pombeiro A.J.L. Adv. Synth. Catal. 2006, 348, 159-174.
Slaughter L.M., Collman J.P., Eberspacher T.A., Brauman J.I., Inorg. Chem. 2004, 43,
5198-5204.
43.
44.
45.
46.
47.
48.
49.
50.
Fenton H.S.H., J. Chem. Soc.,Trans., 1894, 65, 899-910.
Shul’pin G.B., J. Mol. Catal. A: Chem., 2002, 189, 39-66.
Shul’pin G.B., Mini-Rev. Org., 2009, 6, 95-104.
Hikichi S., Okuda H., Ohzu Y. and Akita M. Angew. Chem., Int. Ed., 2009, 48, 188-191.
Nagataki T., Tachi Y. and Itoh S. Chem. Commun., 2006, 4016-4018.
Nagataki T., Ishii K., Tachi Y. and Itoh S. Dalton Trans., 2007, 1120-1128.
Hitomi Y., Furukawa S., Higuchi M., Shishido T., Tanaka T. J. Mol. Catal. A: Chem, 2008,
288, 83-86.
51.
52.
53.
Hikichi S., Hanaue K., Fujimura T., Okuda H., Nakazawa J., Ohzu Y., Kobayashi C. and
Akita M. Dalton Trans., 2013, 42, 3346-3356.
Balamurugan M., Mayilmurugan R, Suresh E., Palaniandavar M., Dalton Trans. 2011, 40,
9413-9424.
Sankaralingam M., Balamurugan M., Palaniandavar M., Vadivelu P. and Suresh C.H.
Chem. Eur. J. 2014, 20, 11346-11361.
54.
55.
56.
Sheldrick G.M., Acta Crystallogr., Sect. A: Fundam. Crystallogr. , 2008, 64, 112-122.
Spek A.L., J. Appl. Crystallogr. , 2003, 36, 7–13.
Farrugia L.J., J. Appl. Crystallogr., 1997, 30, 565-566.
18