Homogeneous oxidation reactions catalysed by in situ-generated triazolylidene copper(I) complexes

Article abstract of DOI:10.1007/s11243-018-0278-5

Abstract: Four new Cu complexes bearing triazolylidene ligands 1-(R)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-yl: R = phenyl (2a), mesitylenyl (2b), propyl (2c), hexyl (2d) (NHC) were synthesised in high yields. Characterisation by spectroscopic and analytical methods confirmed the molecular composition of the complexes as NHC–Cu-I. The complexes 2(a–d) bearing NHC wingtip variations were tested as in situ-generated catalysts for homogeneous oxidation catalysis with H₂O₂ as oxidant. The in situ technique was adopted for ease of application and to circumvent the poor stability of the complexes in solution. The results showed that the NHC–Cu-I complexes are capable of initiating oxidation reactions, yielding ketones/aldehydes as dominant products for the oxidation of alkanes under optimised reaction conditions, with complexes bearing aliphatic N-substituents showing the highest catalytic activities. Oxidation of toluene with 2c resulted in a mixture of benzaldehyde and benzyl alcohol as the main products. Also, 2c catalysed the oxidation of n-octane, yielding a mixture of mainly C-8 oxidation products with over 75% selectivity for the isomeric octanones. Analysis of regioselectivity indicated that the internal Csp3–H bonds of n-octane [especially C(2)] are more reactive than the terminal ones. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11243-018-0278-5

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0278-5
Homogeneous oxidation reactions catalysed by in situ‑generated
triazolylidene copper(I) complexes
Siyabonga G. Mncube1 · Muhammad D. Bala1
Received: 28 August 2018 / Accepted: 4 October 2018
© Springer Nature Switzerland AG 2018
Abstract
Four new Cu complexes bearing triazolylidene ligands 1-(R)-3-methyl-4-phenyl-1H-1,2,3-triazol-3-ium-5-yl: R=phenyl
(2a), mesitylenyl (2b), propyl (2c), hexyl (2d) (NHC) were synthesised in high yields. Characterisation by spectroscopic and
analytical methods confrmed the molecular composition of the complexes as NHC–Cu-I. The complexes 2(a–d) bearing
NHC wingtip variations were tested as in situ-generated catalysts for homogeneous oxidation catalysis with H2O2 as oxidant.
The in situ technique was adopted for ease of application and to circumvent the poor stability of the complexes in solution.
The results showed that the NHC–Cu-I complexes are capable of initiating oxidation reactions, yielding ketones/aldehydes
as dominant products for the oxidation of alkanes under optimised reaction conditions, with complexes bearing aliphatic
N-substituents showing the highest catalytic activities. Oxidation of toluene with 2c resulted in a mixture of benzaldehyde
and benzyl alcohol as the main products. Also, 2c catalysed the oxidation of n-octane, yielding a mixture of mainly C-8 oxi-
dation products with over 75% selectivity for the isomeric octanones. Analysis of regioselectivity indicated that the internal
Csp3–H bonds of n-octane [especially C(2)] are more reactive than the terminal ones.
Introduction
steric and electronic tuning through variation of the wingtip
N-substituents of the azole ring [4]. Since the frst cationic
di-carbene (NHC)2–Cu complex was reported by Arduengo
[5] and later application of NHC–Cu complexes in cataly-
sis by Woodward et al. [6], a great deal of catalytic and
structural work in this area has continued to build on those
foundations [7]. In spite of all the interest, the handling of
NHC–Cu complexes is non-trivial, and concerns on the rela-
tive stability of “free” NHC–Cu have resulted in the develop-
ment of in situ catalytic approaches as alternative methods.
This technique was frst reported in 2001 by Woodward, [6]
from which a number of reports soon followed, all signify-
ing that copper salts make better catalysts in the presence of
NHC ligands [8–10]. Hence, in this report we present new
NHC–CuI catalysts generated in situ for the oxidation of a
range of substrates under mild reaction conditions.
The oxidation of saturated substrates via activation of unre-
active bonds under mild catalytic conditions is still a chal-
lenging global problem to both industry and academia [1].
Over the years, frst-row transition metal complexes have
emerged as cheap and efective oxidation catalysts that are
capable of operating under mild reaction conditions [2]. In
particular, copper complexes bearing bulky organic ligands
have shown great potential as homogeneous catalysts for the
efcient oxidation of a wide range of substrates [3].
With regard to choice of ligand, N-heterocyclic carbenes
(NHC) have become very prominent in organometallic
chemistry and their metal complexes have made impor-
tant contributions in catalysis. This is due to their ability to
form stable covalent metal–ligand bonds and their ease of
Results and discussion
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0278-5) contains
supplementary material, which is available to authorized users.
Synthesis of the triazolylidene Cu(I) complexes
* Muhammad D. Bala
We adopted the method of Son et al. [11] for the synthesis
of the NHC–Cu complexes (2, Scheme 1). This is a very
simple method akin to the well-established Ag2O route for
bala@ukzn.ac.za
1
School of Chemistry and Physics, University of KwaZulu-
Natal, Private Bag X54001, Durban 4000, South Africa
Vol.:(0123456789)
1 3

Transition Metal Chemistry
which all supported formation of complexes of the type
NHC–Cu-I.
Ph
Ph
N
N
N
N
I
Cu2O
Attempts to crystallise 2 failed due to decomposition
in solution to reform 1 and unidentifed insoluble copper
salts (vide infra). On this basis, a study of the stability and
decomposition rate of 2 was conducted and monitored after
indicated time (Fig. 1). Complex 2a was used as representa-
tive for the study. In the solid state, complex 2a was found to
be air-sensitive, transforming completely into a green insol-
uble solid within a few minutes. Under a N2 atmosphere,
no noticeable decomposition was observed, however, after
16 h in the same atmosphere, a degree of decomposition
was recorded.
Cu
I
dioxane
60 °C
8 h
N
N
R
R
1(a-d)
2 (a-d)
R = C6H5 (a); C6H3(CH3)3 (b); C3H7 (c);
C6H13 (d).
Scheme 1 Synthesis of the NHC–Cu-I complexes (2a–d) by the
direct reaction of NHC (1a–d) with Cu2O
In solution, the decomposition of 2a noticeably depends
on the degree of solvent (CDCl3) dryness and time in solu-
tion. A time-dependent study was set up, and the result is
shown in Fig. 1. The progress of the decomposition was
the synthesis of NHC–metal complexes. Hence, the direct
reaction of the proligands 1(a–d) with Cu2O in dioxane
correspondingly led to the formation of complexes 2(a–d).
Workup procedure and isolation of the complexes in pure
form are also very straightforward and uncomplicated.
Hence, at the end of each run, solids (mainly insoluble, unre-
acted Cu2O) were simply fltered of and the clear fltrate
evaporated to dryness, yielding circa 70% of analytically
pure 2(a–d). All the complexes were isolated and charac-
terised by 1H and 13C NMR spectroscopy, HRMS and EA,
1
monitored by H-NMR spectroscopy (by monitoring the
signal of the triazolium C-2 proton, marked d in Fig. 1).
The data obtained showed strong time dependence of the
decomposition of 2a, such that up to 2 h in solution there
was not much decomposition, however, after 8 h, about 10%
of 2a was transformed back into 1a, the starting triazolium
salt. No signs of 2a were observed after 24 h, suggesting
Fig. 1 Time-dependent 1H
a
Ph
NMR spectral changes of 2a in
Ph
c
CDCl3
N
N
I
CDCl3
d
Cu
I
H
N
N
N
N
b
Ph
Ph
1a
2a
1 3

Transition Metal Chemistry
complete decomposition. This was also confrmed by 13C-
NMR (loss of the low feld resonance at 168 ppm). The poor
stability of complexes 2(a–d) may be synthetically exploited
as carbene transfer agents en route to the synthesis of NHC-
transition metal complexes, similar to the well-established
Ag2O route.
Table 1 Oxidation of cyclohexane with in situ-generated 2
OH
O
2 (a-d)
H2O2
CH3CN, 60 °C
Cy-OH
Cy-H
Cy=O
Oxidation of cyclohexane
A preliminary attempt to isolate the NHC–Cu-I complexes
and apply them as molecular catalysts for oxidation reac-
tions was unsuccessful due to their decomposition, lead-
ing us to adopt in situ catalytic methods instead. Hence,
the in situ-generated NHC–Cu-I complexes were evaluated
as catalysts for oxidation catalysis. The frst set of results
were obtained on cyclohexane as a model substrate for the
optimisation of reaction conditions, and the results are pre-
sented in Table 1. Entries 1 and 2 confrm the necessity for
a transition metal in this catalytic study as no products were
observed for a control experiment in the absence of Cu2O
(entry 1, Table 1). However, addition of the metal Cu2O
revealed vigorous oxidation products (entry 2) within 4 h
of reaction, indicating its signifcance as the catalytic centre
in the activation of cyclohexane. Furthermore, as revealed
by entry 3, it takes a combination of both Cu2O and 1a to
induce 27% cyclohexane conversion to the product mixture
of cyclohexanone–cyclohexanol, so-called ketone–alcohol
(K–A) oil. The K–A oil is an important feedstock for the
industrial production of nylon. These results point to the
direct infuence of the Cu/triazolium ligand combination in
the catalytic process.
Entrya Catalyst
Temperature Conversion Selectivity
NHC:Cu2O
(mol%)b
(mol%)c
Cy=O Cy–OH
1
2
1:0
0:1
50
50
50
80
80
80
80
80
80
80
80
80
–
–
–
12
27
31
33
31
11
14
7
85
39
32
48
28
42
39
28
47
37
39
15
61
68
52
72
58
61
72
53
63
61
3
4
1:1
1:1
5
6
1:2
2:1
7d
8e
9f
10g
11h
12i
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
1:1.1
21
39
38
aReaction conditions (unless stated otherwise): Substrate (1.0 mol),
2(a) (10 mmol, 1 mol% vs. substrate), H2O2 (10.0 mol), MeCN
(5 mL), indicated temperature, 24 h
bConversion=conc. products/initial conc. of cyclohexane×100
cSelectivity=moles of CyO/(CyOH+CyO)×100
dHCl added (1.0 mol)
eAcetic acid added (1.0 mol)
The extent of substrate conversion was slightly improved
when the reaction temperature was increased to 80 °C, sug-
gesting the positive efect of temperature in this catalytic
reaction (entry 4). The study was limited to 80 °C due to
volatility of both the substrate and the solvent from the
unpressurised reaction set-up. Increasing the amount of
Cu2O showed no positive efects on catalytic activity; how-
ever, a notable increase in ketone production was observed
(entry 5), raising the K/A ratio.
fTBHP used (instead of H2O2)
g2(b)
h2(c)
i2(d)
In an efort to relate the kinetics of the oxidation reaction
with the lifespan of the generated catalyst, a time-dependent
study was conducted, with the results presented in Fig. 2.
It is clear that the catalytic conversion of cyclohexane to
products is time-dependent with no induction period and the
in situ-generated catalyst achieved a maximum production of
39% at 20 h reaction time. The initial rate tapered of after
4 h of reaction and then levelled of after a period of 16 h
which matches the onset of gradual catalyst decomposition
observed by time-dependent NMR study (Fig. 1).
On comparison of structure/activity relationships of the
in situ-generated catalysts in this series, it is clear that due
to steric reasons, the two complexes bearing bulky aromatic
wingtip N-substituents 2(a–b) showed lower catalytic activi-
ties (31 and 21% conversions, entries 5 and 10) than 2(c–d)
functionalised with aliphatic N-substituents (39 and 38%
conversions, entries 11 and 12), respectively.
The addition of acids as promoters has been shown to
improve substrate conversion in numerous oxidation reac-
tions [12–14]. However, in this study, addition of acids
(entries 7 and 8) rather retarded the oxidation reaction
resulting in poor conversions, which is most likely due to
catalyst decomposition in the harsh acidic environments.
It is also important to determine the active species in
this catalytic study; hence, a stoichiometric amount of a
radical scavenger (diphenylamine, Ph2NH) was added to
the cyclohexane oxidation catalysis set-up. Ph2NH was
observed to suppress the catalyst activity, indicating the
1 3

Transition Metal Chemistry
Fig. 2 Efect of reaction time
on cyclohexane oxidation with
in situ-generated 2a [black
diamond] conversion; [Red
triangle] conversion (A) when
Ph2NH radical scavenger was
added. Reaction conditions:
same as in Table 1. (Color
fgure online)
45
40
35
30
25
20
15
10
5
0
0
10
20
30
40
50
Time/h
Table 2 Comparative representation of oxidation of cyclohexane by a
variety of Cu-based catalyst systemsa
Oxidation of toluene
Toluene is an aromatic hydrocarbon commonly adopted as
a safer and environmentally benign alternative to benzene
and widely used as an organic solvent in laboratory and
industrial applications. Hence, it is important to study its
stability and tolerance of oxidising environments in the
presence of transition metal salts with proven potential to
activate the C–H bonds of hydrocarbon substrates. Also,
the product of toluene oxidation, benzaldehyde is a desir-
able starting material due to its importance as a building
block for the fne chemicals and pharmaceuticals industry.
However, the oxidation of toluene is reported to sufer
from low selectivity to the desired aldehyde due to the
possibility of dual activation of both the methyl and/or
aromatic C–H bonds depending on various reaction condi-
tions (type of catalyst, solvent, oxidant, etc.). In this study
with 2c as catalyst, the oxidation of toluene resulted in a
mixture of oxidation products (total conversion up to 11%)
with benzaldehyde (PhCHO, 62%) and benzyl alcohol
(PhCH2OH, 29%) as the main products and o/m/p-cresols
as minor products, which is inline with literature reports
for similar studies [18–20].
Entry
Catalytic system
Conversion References
(mol%)
1b
2
[CuI(bpy)2]/pyridine
Cu powder/pyridine/Ac
([Cu4(tea)–(BOH)4(m–O)]2+
CuI/TMPA
15
5
38
53
56
39
[14]
[15]
[16]
3c
4d
5e
6
)
[17]
CuI/(R,R)-BPBP
2c
[18]
This work
aAll reactions were performed in MECN using H2O2 as an oxidant
under mild reaction conditions
bbpy 2,2′-bipyridine
ctea trimethylamine
dTMPA (tris(2-pyridylmethyl)amine)
eBPBP ((2R,2′R)-1,1′-bis(2-pyridylmethyl)-2,2′-bipyrrolidine)
involvement of radical pathways in the oxidation reaction.
Hence, we conclude that the in situ-generated catalytic
systems based on a mixture of NHC/Cu2O rely on Cu+/2+
as the active species generated in solution from the gradual
decomposition of 2(a–d) which act as reservoirs of Cu+
ions.
To put these results into context with closely related lit-
erature, comparative reports are summarised in Table 2. The
group of Sawyer [14] reported good conversions of up to
15% with [CuI(bpy)2]/pyridine catalyst mixture which fol-
lowed the very low conversions (up to 5%) obtained by Bar-
ton et al. [15], using combined Cu powder/pyridine/acetic
acid catalyst mixture. Having observed steric infuence of
ancillary ligands in the oxidation of cyclohexane, Pombeiro
et al. [16] reported efcient conversions (up to 38%) for
systems using a tetranuclear Cu cluster (entry 3). Recently,
Siegler and Garcia-Bosch [17] reported a highly active and
selective copper complex (entry 5) as a catalyst for the mild
peroxidative activation and functionalisation of cyclohexane
(conversions up to 56%) to the K–A oil product mixture.
Oxidation of n‑octane
The results obtained with 2c as the catalyst produced 22%
total conversion of the substrate selectively to a mixture
of C-8 oxidation products with 80% total selectivity to
octanones. GC analysis of the product stream indicated no
evidence of cracking or heavy products. Further analysis of
the regioselectivity parameter showed that internal carbons
[especially C(2)] are more reactive than terminal ones and
only 8% terminal C(1) activation products were observed.
Similarly, 2-octanone (ca. 38%) is the most abundant prod-
uct in the stream. This is inline with observations that are
numerously reported in the literature indicating 2-octanone
as the more probable product of n-octane oxidation [21, 22].
1 3

Transition Metal Chemistry
Table 3 Oxidation of alcohols catalysed by 2c with H2O2 as oxidanta
reactions. In this study, we present a simplifed mechanism
(Scheme 2) that accounts for the ketone and alcohol prod-
ucts we observed. The proposed mechanism involves the
interaction of catalytically active Cu(I) species with H2O2
to generate a hydroxyl radical (HO·) which then activates
the substrate alkane (R-H, 4) via proton abstraction, lead-
ing to a highly activated alkyl radical (R·) which further
reacts with molecular oxygen (O2) present in the reaction
medium to form an alkyl peroxyl radical (ROO·, 5) [17]. We
believe, this is the rate-determining step that is curtailed by a
radical scavenger such as Ph2NH. The alkyl peroxyl radical
may undergo dismutation to yield the alcohol and ketone
products or undergo reduction into the major product, alkyl
hydroperoxide (ROOH, 6).
Entry
Substrate
Product
Conversion (%)
OH
O
1
2
3
88
91
73
OH
O
O
OH
OH
O
4
5
76
61
O
OH
OH
O
O
6
7
8
35
38
68
OH
O
The ROOH decomposes to the corresponding alcohol
(R-OH) and ketone (R=O, 7) products depending on reac-
tion conditions, and it is also often reduced to R-OH and
R=O by the addition of suitable reducing agents such as
PPh3, in a method developed by Shulpin et al. [29].
OH
aReaction conditions: same as in Table 1.
Conclusion
Oxidation of alcohols
The chemistry of a series of easily prepared NHC–Cu-I com-
plexes was presented. These complexes are unstable both
in the solid state and in solution, decomposing within a 24
h period. We believe that their stability may be improved
by the utilisation of bulky and electron-donating wingtip
substituents such as mesitylenyl or adamantyl instead of sim-
ple methyl and phenyl groups. As in situ-generated copper
catalysts, all the complexes were efective for the homogene-
ous oxidation of alkanes and alcohols under mild reaction
conditions and at low concentrations. A reaction pathway
involving alkyl peroxyl radicals was proposed because of the
Finally, complex 2c was tested as a catalyst for the in situ
homogeneous oxidation of alcohols to the corresponding
carbonyl products using H2O2 under optimised reaction
conditions. The results summarised in Table 3 revealed that
phenolic and benzylic alcohols are more reactive than the
aliphatic variants. This observation, which is attributed to
activation of the C–OH bond in the cyclic hydrocarbons and
aromatic systems, is consistent with related results reported
in the literature. [23–27] Also, the oxidation of cyclohexanol
(entry 2) with 2c provided the highest conversion (91%).
Oxidation of linear alcohols resulted in lower conversions
(entries 6, 7 and 8), whilst the primary alcohols are much
less reactive than the secondary (2-octanol), which is con-
sistent with reported observations suggesting that reactions
at the secondary carbon positions of aliphatic hydrocarbons
are more facile than terminal/primary carbons.
R-H + H2O2
(4)
R-OH + R=O
(7)
NHC-CuI
(2)
Mechanistic considerations
On the basis of all the experimental data presented herein,
it is proposed that these alkane oxidation reactions progress
via the formation of alkyl hydroperoxide (ROOH) as the
major product. As was shown in this study, the addition
of a radical scavenger (Ph2NH) inhibited the oxidation of
cyclohexane (Fig. 2), afrming a radical pathway for the oxi-
dation process. The full understanding of the mechanistic
steps involved in this pathway has been the main subject of
many research publications [17, 28–31]. Although research-
ers are still debating some of the mechanistic steps, a radi-
cal mechanism is now well established for these oxidation
NHC-CuII-OH
(3)
R-OH + R=O
ROO.
ROOH
(5)
(6)
Scheme 2 Simplifed reaction mechanism for the oxidation of an
alkane by the NHC–CuI catalysts
1 3

Transition Metal Chemistry
1
negative efects of a radical scavenger and the product selec-
tivity of the systems. Given the promising catalytic results
obtained with these new NHC–Cu-I systems, further work
will be devoted to the development of multidentate NHC
ligands (with potential auxiliary donor atoms) also aimed
at achieving bench-top stability and furthering the range of
applications for the complexes.
(2b) Greenish solid product (0.38 g, 68%). H-NMR
(CDCl3, 400 MHz): δ 7.5–7.7 (m, 5H, Ar), 6.0 (s, 2H, Ar),
4.3(s, 3H, N–CH3), 2.0(bs, 9H, 3CH3). 13C{1H}-NMR
(CDCl3, 100.6 MHz): δ 166.5, 142.7, 131.9, 131.5, 130.5,
129.8, 129.6, 129.6, 129.3, 128.0, 121.9, 38.0, 22.3. TOF
MS+ (ESI) m/z for C18H19CuN3 [M+-I]: calculated: 340.0,
found: 340.0. Anal. (%) calc. for (C18H19CuIN3, 466.9): C,
46.2; H, 4.0; N, 8.9; found: C, 46.2; H, 4.1; N, 9.0.
(2c) Greenish solid product (0.53 g, 73%). 1H-NMR (CDCl3
400 MHz): δ 7.2–7.4 (m, 5H, Ar), 4.7 (m, 2H, CH2), 4.2 (s,
3H, N–CH3), 1.9 (m, 2H, CH2), 0.9 (t, 3H, CH3). 13C{1H}-
NMR (CDCl3, 100.6 MHz): δ 165.0, 143.3, 131.9, 130.9,
129.7, 129.6, 129.4, 121.8, 38.7, 27.4, 13.9. TOF MS+ (ESI)
m/z for C12H15CuN3 [M+-I]: calculated: 264.0, found: 264.1.
Anal. (%) calc. for (C12H15CuIN3, 390.9): C, 36.8; H, 3.9; N,
10.7; found: C, 36.8; H, 3.9; N, 10.7.
Experimental section
General
All the reagents were purchased from Sigma-Aldrich and
used as received. All the solvents (acetonitrile, tetrahy-
drofuran, diethyl ether and hexanes) were purchased from
Merck and purifed using a commercially available MBraun
MB-SP Series solvent purifcation system equipped with
activated alumina columns. Unless otherwise stated, all
syntheses were performed under a nitrogen atmosphere
using standard Schlenk techniques. Triazoles and their
corresponding salts were synthesised as described in pub-
lished literature and characterisation data are consistent
with reported literature values [32, 33]. NMR spectra were
recorded on a Bruker Avance 400 MHz spectrometer oper-
ated at ambient temperature with δ values reported in ppm
referenced to Me4Si as the internal standard for both 1H and
13C NMR data. Elemental analysis (CHN) was performed
on a LECO CHNS elemental analyser, and the mass spec-
trometric data were collected on a Bruker Daltonics (micro
TOF) instrument.
(2d) Greenish solid product (0.86 g, 75%). 1H-NMR (CDCl3
400 MHz): δ 7.6–7.8 (m, 5H, Ar), 4.7 (m, 2H, CH2), 4.3 (s,
3H, N–CH3), 2.1 (m, 2H, CH2) 1.4–1.5 (m, 6H, 3CH2), 0.9
(t, 3H, CH3). 13C{1H}-NMR (CDCl3, 100.6 MHz): δ 166.8,
144.5, 132.90, 130.8, 130., 130.02, 124.0, 55.3, 39.7, 32.3,
30.35, 27.0, 23.5, 14.4. TOF MS+ (ESI) m/z for C15H21CuN3
[(M+-I)+2Li]: calculated: 320.1, found: 320.2. Anal. (%)
calc. for (C15H21CuIN3 +2CH3CN, 515.1): C, 44.2; H, 5.3;
N, 13.6; found: C, 44.2; H, 5.3; N, 11.5.
General procedure for catalytic reactions
The oxidants H2O2 (30%) and TBHP (70%) were, respec-
tively, purchased from Aldrich and DLD Scientifc and used
as supplied. A typical oxidation reaction was carried out in
a 2-neck round-bottom fask ftted with an efcient refux
condenser operating in air as follows: an appropriate amount
of the precursor salt 1(a–d) was treated with a stoichiomet-
ric amount of Cu2O in CH3CN (5 mL) for the indicated
temperature and time. The substrate and oxidant were then
added, and the mixture was heated at the indicated tempera-
ture for the required time. The product was analysed after
the required time period, an aliquot of sample was removed
using a Pasteur pipette and fltered through a cotton wool
plug, after which 0.5 μL of the aliquot was injected into the
GC for analysis and quantifcation. 2,4,6-Trichlorobenzene
was used as the internal standard, and all experiments were
conducted with an Agilent Technology 6820 GC System
equipped with a fame ionisation detector (FID), and an
Agilent DB-Wax column with a length of 30 metres, inner
diameter of 0.25 mm and a thickness of 0.25 mm.
General procedure for synthesis of the complexes
(2a–d)
Procedure for the synthesis of NHC–Cu complexes [11]:
triazolium salt (0.3 mmol) and Cu2O (0.4 mmol) in 4 mL
of dioxane were heated at 60 °C for 8 h. The reaction mix-
ture was then cooled to room temperature, and the unreacted
Cu2O was removed by fltration. The remaining solvent was
evaporated to yield precipitates of the complex. Characteri-
sation data are:
(2a) White solid product (0.41 g, 71%). 1H-NMR (CDCl3,
400 MHz): δ 7.1–7.6 (bm, 10H, Ar), 4.7 (s, 3H, N-CH3),
13C{1H}-NMR (CDCl3, 100.6 MHz): δ 168.4, 148.5,
129.9, 129.7, 129.1, 128.7, 128.3, 126.8, 125.0, 121.0,
120.5, 117.6, 37.9. TOF MS+ (ESI) m/z for C15H13CuN3
[M+-I]: calculated: 298.0, found: 298.0. Anal. (%) calc. for
(C15H13CuIN3, 425.7): C, 42.3; H, 30.0; N, 9.8; found: C,
42.3; H, 3.0; N, 9.8.
Acknowledgements This project is generously supported by c*change
PAR program, the National Research Foundation and the University of
KwaZulu-Natal for which we are grateful (Grant No. PAR08).
1 3

Transition Metal Chemistry
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